Apert syndrome is characterized by the presence of multisuture craniosynostosis, midface retrusion, and syndactyly of the hands with fusion of the second through fourth nails. Almost all affected individuals have coronal craniosynostosis, and a majority also have involvement of the sagittal and lambdoid sutures. The midface in Apert syndrome is underdeveloped as well as retruded; a subset of affected individuals have cleft palate. The hand in Apert syndrome always includes fusion of the middle three digits; the thumb and fifth finger are sometimes also involved. Feeding issues, dental abnormalities, hearing loss, hyperhidrosis, and progressive synostosis of multiple bones (skull, hands, feet, carpus, tarsus, and cervical vertebrae) are also common. Multilevel airway obstruction may be present and can be due to narrowing of the nasal passages, tongue-based airway obstruction, and/or tracheal anomalies. Nonprogressive ventriculomegaly is present in a majority of individuals, with a small subset having true hydrocephalus. Most individuals with Apert syndrome have normal intelligence or mild intellectual disability; moderate-to-severe intellectual disability has been reported in some individuals. A minority of affected individuals have structural cardiac abnormalities, true gastrointestinal malformations, and anomalies of the genitourinary tract.

Classic ataxia-telangiectasia (A-T) is characterized by progressive cerebellar ataxia beginning between ages one and four years, oculomotor apraxia, choreoathetosis, telangiectasias of the conjunctivae, immunodeficiency, frequent infections, and an increased risk for malignancy, particularly leukemia and lymphoma. Individuals with A-T are unusually sensitive to ionizing radiation. Non-classic forms of A-T have included adult-onset A-T and A-T with early-onset dystonia.

Nevoid basal cell carcinoma syndrome (NBCCS) is characterized by the development of multiple jaw keratocysts, frequently beginning in the second decade of life, and/or basal cell carcinomas (BCCs) usually from the third decade onward. Approximately 60% of individuals have a recognizable appearance with macrocephaly, frontal bossing, coarse facial features, and facial milia. Most individuals have skeletal anomalies (e.g., bifid ribs, wedge-shaped vertebrae). Ectopic calcification, particularly in the falx, is present in more than 90% of affected individuals by age 20 years. Cardiac and ovarian fibromas occur in approximately 2% and 20% of individuals respectively. Approximately 5% of all children with NBCCS develop medulloblastoma (primitive neuroectodermal tumor), generally the desmoplastic subtype. The risk of developing medulloblastoma is substantially higher in individuals with an SUFU pathogenic variant (33%) than in those with a PTCH1 pathogenic variant (<2%). Peak incidence is at age one to two years. Life expectancy in NBCCS is not significantly different from average.

Chediak-Higashi syndrome (CHS) is characterized by partial oculocutaneous albinism, immunodeficiency, and a mild bleeding tendency. Approximately 85% of affected individuals develop the accelerated phase, or hemophagocytic lymphohistiocytosis, a life-threatening, hyperinflammatory condition. All affected individuals including adolescents and adults with atypical CHS and children with classic CHS who have successfully undergone allogenic hematopoietic stem cell transplantation (HSCT) develop neurologic findings during early adulthood.

Crouzon syndrome is an autosomal dominant disorder characterized by craniosynostosis causing secondary alterations of the facial bones and facial structure. Common features include hypertelorism, exophthalmos and external strabismus, parrot-beaked nose, short upper lip, hypoplastic maxilla, and a relative mandibular prognathism (Reardon et al., 1994; Glaser et al., 2000).

Cri-du-chat syndrome was first described by Lejeune et al. (1963) as a hereditary congenital syndrome associated with deletion of part of the short arm of chromosome 5. The deletions can vary in size from extremely small and involving only band 5p15.2 to the entire short arm. Although the majority of deletions arise as new mutations, approximately 12% result from unbalanced segregation of translocations or recombination involving a pericentric inversion in one of the parents.

Focal dermal hypoplasia is a multisystem disorder characterized primarily by involvement of the skin, skeletal system, eyes, and face. Skin manifestations present at birth include atrophic and hypoplastic areas of skin; cutis aplasia; fat nodules in the dermis manifesting as soft, yellow-pink cutaneous nodules; and pigmentary changes. Verrucoid papillomas of the skin and mucous membranes may appear later. The nails can be ridged, dysplastic, or hypoplastic; hair can be sparse or absent. Limb malformations include oligo-/syndactyly and split hand/foot. Developmental abnormalities of the eye can include anophthalmia/microphthalmia, iris and chorioretinal coloboma, and lacrimal duct abnormalities. Craniofacial findings can include facial asymmetry, notched alae nasi, cleft lip and palate, and pointed chin. Occasional findings include dental anomalies, abdominal wall defects, diaphragmatic hernia, and renal anomalies. Psychomotor development is usually normal; some individuals have cognitive impairment.

Hallermann-Streiff syndrome is characterized by a typical skull shape (brachycephaly with frontal bossing), hypotrichosis, microphthalmia, cataracts, beaked nose, micrognathia, skin atrophy, dental anomalies, and proportionate short stature (Hallermann, 1948; Streiff, 1950; Francois, 1958). Mental retardation is present in a minority of cases (Gorlin et al., 1990).

Incontinentia pigmenti (IP) is a disorder that affects the skin, hair, teeth, nails, eyes, and central nervous system; it occurs primarily in females and on occasion in males. Characteristic skin lesions evolve through four stages: I. Blistering (birth to age ~4 months). II. Wart-like rash (for several months). III. Swirling macular hyperpigmentation (age ~6 months into adulthood). IV. Linear hypopigmentation. Alopecia, hypodontia, abnormal tooth shape, and dystrophic nails are observed. Neovascularization of the retina, present in some individuals, predisposes to retinal detachment. Neurologic findings including seizures, intellectual disability, and developmental delays are occasionally seen.

A large brown, blue, or gray hamartoma of dermal melanocytes, usually on the shoulder and upper arm that is most commonly found in Asian populations and in females. It is sometimes associated with sensory changes in the involved skin area, but very rarely becomes cancerous.

Leigh syndrome is a clinical diagnosis based primarily on characteristic brain imaging findings associated with progressive and severe neurodegenerative features with onset within the first months or years of life, sometimes resulting in early death. Affected individuals usually show global developmental delay or developmental regression, hypotonia, ataxia, dystonia, and ophthalmologic abnormalities, such as nystagmus or optic atrophy. The neurologic features are associated with the classic findings of T2-weighted hyperintensities in the basal ganglia and/or brainstem on brain imaging. Leigh syndrome can also have detrimental multisystemic affects on the cardiac, hepatic, gastrointestinal, and renal organs. Biochemical studies in patients with Leigh syndrome tend to show increased lactate and abnormalities of mitochondrial oxidative phosphorylation (summary by Lake et al., 2015). Genetic Heterogeneity of Nuclear Leigh Syndrome Leigh syndrome is a presentation of numerous genetic disorders resulting from defects in the mitochondrial OXPHOS complex. Accordingly, the genes implicated in Leigh syndrome most commonly encode structural subunits of the OXPHOS complex or proteins required for their assembly, stability, and activity. Mutations in both nuclear and mitochondrial genes have been identified. For a discussion of genetic heterogeneity of mitochondrial Leigh syndrome, see MILS (500017). Nuclear Leigh syndrome can be caused by mutations in nuclear-encoded genes involved in any of the mitochondrial respiratory chain complexes: complex I deficiency (see 252010), complex II deficiency (see 252011), complex III deficiency (see 124000), complex IV deficiency (cytochrome c oxidase; see 220110), and complex V deficiency (see 604273) (summary by Lake et al., 2015). Some forms of combined oxidative phosphorylation deficiency (COXPD) can present as Leigh syndrome (see, e.g., 617664). Leigh syndrome may also be caused by mutations in components of the pyruvate dehydrogenase complex (e.g., DLD, 238331 and PDHA1, 300502). Deficiency of coenzyme Q10 (607426) can present as Leigh syndrome.

Dominant cystoid macular dystrophy (DCMD) is a progressive retinal dystrophy characterized primarily by early-onset cystoid fluid collections in the neuroretina (summary by Saksens et al., 2015).

FBN1-related Marfan syndrome (Marfan syndrome), a systemic disorder of connective tissue with a high degree of clinical variability, comprises a broad phenotypic continuum ranging from mild (features of Marfan syndrome in one or a few systems) to severe and rapidly progressive neonatal multiorgan disease. Cardinal manifestations involve the ocular, skeletal, and cardiovascular systems. Ocular findings include myopia (>50% of affected individuals); ectopia lentis (seen in approximately 60% of affected individuals); and an increased risk for retinal detachment, glaucoma, and early cataracts. Skeletal system manifestations include bone overgrowth and joint laxity; disproportionately long extremities for the size of the trunk (dolichostenomelia); overgrowth of the ribs that can push the sternum in (pectus excavatum) or out (pectus carinatum); and scoliosis that ranges from mild to severe and progressive. The major morbidity and early mortality in Marfan syndrome relate to the cardiovascular system and include dilatation of the aorta at the level of the sinuses of Valsalva (predisposing to aortic tear and rupture), mitral valve prolapse with or without regurgitation, tricuspid valve prolapse, and enlargement of the proximal pulmonary artery. Severe and prolonged regurgitation of the mitral and/or aortic valve can predispose to left ventricular dysfunction and occasionally heart failure. With proper management, the life expectancy of someone with Marfan syndrome approximates that of the general population.

Marinesco-Sjögren syndrome (MSS) is characterized by cerebellar ataxia with cerebellar atrophy, dysarthria, nystagmus, early-onset (not necessarily congenital) cataracts, myopathy, muscle weakness, and hypotonia. Additional features may include psychomotor delay, hypergonadotropic hypogonadism, short stature, and various skeletal abnormalities. Children with MSS usually present with muscular hypotonia in early infancy; distal and proximal muscular weakness is noticed during the first decade of life. Later, cerebellar findings of truncal ataxia, dysdiadochokinesia, nystagmus, and dysarthria become apparent. Motor function worsens progressively for some years, then stabilizes at an unpredictable age and degree of severity. Cataracts can develop rapidly and typically require lens extraction in the first decade of life. Although many adults have severe disabilities, life span in MSS appears to be near normal.

The X-linked otopalatodigital (X-OPD) spectrum disorders, characterized primarily by skeletal dysplasia, include the following: Otopalatodigital syndrome type 1 (OPD1). Otopalatodigital syndrome type 2 (OPD2). Frontometaphyseal dysplasia type 1 (FMD1). Melnick-Needles syndrome (MNS). Terminal osseous dysplasia with pigmentary skin defects (TODPD). In OPD1, most manifestations are present at birth; females can present with severity similar to affected males, although some have only mild manifestations. In OPD2, females are less severely affected than related affected males. Most males with OPD2 die during the first year of life, usually from thoracic hypoplasia resulting in pulmonary insufficiency. Males who live beyond the first year of life are usually developmentally delayed and require respiratory support and assistance with feeding. In FMD1, females are less severely affected than related affected males. Males do not experience a progressive skeletal dysplasia but may have joint contractures and hand and foot malformations. Progressive scoliosis is observed in both affected males and females. In MNS, wide phenotypic variability is observed; some individuals are diagnosed in adulthood, while others require respiratory support and have reduced longevity. MNS in males results in perinatal lethality in all recorded cases. TODPD, seen only in females, is characterized by a skeletal dysplasia that is most prominent in the digits, pigmentary defects of the skin, and recurrent digital fibromata.

An autosomal dominant inherited condition caused by mutations in the lamin B receptor gene. It is characterized by defects in the neutrophil lobulation, resulting in the presence of dumbbell-shaped neutrophils with bilobed nuclei in the peripheral blood smear.

Prader-Willi syndrome (PWS) is characterized by severe hypotonia and feeding difficulties in early infancy, followed in later infancy or early childhood by excessive eating and gradual development of morbid obesity (unless eating is externally controlled). Motor milestones and language development are delayed. All individuals have some degree of cognitive impairment. A distinctive behavioral phenotype (with temper tantrums, stubbornness, manipulative behavior, and obsessive-compulsive characteristics) is common. Hypogonadism is present in both males and females and manifests as genital hypoplasia, incomplete pubertal development, and, in most, infertility. Short stature is common (if not treated with growth hormone); characteristic facial features, strabismus, and scoliosis are often present.

A condition characterized by recurrent, self-limiting episodes of vomiting associated with intense nausea, pallor, and lethargy. It is commonly a migraine precursor.

Hidrotic ectodermal dysplasia 2, or Clouston syndrome (referred to as HED2 throughout this GeneReview) is characterized by a triad of major clinical features including partial-to-complete alopecia, nail dystrophy, and palmoplantar hyperkeratosis. Sweating is preserved and there are usually no dental anomalies. Sparse scalp hair and dysplastic nails are seen early in life. In infancy, scalp hair is fine, sparse, and brittle. Progressive hair loss may lead to total alopecia by puberty. The nails may be milky white in early childhood; they gradually become dystrophic, thick, and distally separated from the nail bed. Palmoplantar keratoderma may develop during childhood and increases in severity with age. Associated features may include cutaneous hyperpigmentation (particularly over the joints) and finger clubbing. The clinical manifestations are highly variable even within the same family.

Angelman syndrome (AS) is characterized by severe developmental delay or intellectual disability, severe speech impairment, gait ataxia and/or tremulousness of the limbs, and unique behavior with an apparent happy demeanor that includes frequent laughing, smiling, and excitability. Microcephaly and seizures are also common. Developmental delays are first noted at around age six months; however, the unique clinical features of AS do not become manifest until after age one year.

Dubowitz syndrome (DS) is a rare multiple congenital syndrome characterized primarly by growth retardation, microcephaly, distinctive facial dysmorphism, cutaneous eczema, a mild to severe intellectual deficit and genital abnormalities.

Johanson-Blizzard syndrome is an autosomal recessive disorder characterized by poor growth, mental retardation, and variable dysmorphic features, including aplasia or hypoplasia of the nasal alae, abnormal hair patterns or scalp defects, and oligodontia. Other features include hypothyroidism, sensorineural hearing loss, imperforate anus, and pancreatic exocrine insufficiency (summary by Al-Dosari et al., 2008).

Smith-Lemli-Opitz syndrome (SLOS) is a congenital multiple-anomaly / cognitive impairment syndrome caused by an abnormality in cholesterol metabolism resulting from deficiency of the enzyme 7-dehydrocholesterol (7-DHC) reductase. It is characterized by prenatal and postnatal growth restriction, microcephaly, moderate-to-severe intellectual disability, and multiple major and minor malformations. The malformations include distinctive facial features, cleft palate, cardiac defects, underdeveloped external genitalia in males, postaxial polydactyly, and 2-3 syndactyly of the toes. The clinical spectrum is wide; individuals with normal development and only minor malformations have been described.

Sotos syndrome is characterized by a distinctive facial appearance (broad and prominent forehead with a dolichocephalic head shape, sparse frontotemporal hair, downslanting palpebral fissures, malar flushing, long and narrow face, long chin); learning disability (early developmental delay, mild-to-severe intellectual impairment); and overgrowth (height and/or head circumference =2 SD above the mean). These three clinical features are considered the cardinal features of Sotos syndrome. Major features of Sotos syndrome include behavioral problems (most notably autistic spectrum disorder), advanced bone age, cardiac anomalies, cranial MRI/CT abnormalities, joint hyperlaxity with or without pes planus, maternal preeclampsia, neonatal complications, renal anomalies, scoliosis, and seizures.

Classic Saethre-Chotzen syndrome (SCS) is characterized by coronal synostosis (unilateral or bilateral), facial asymmetry (particularly in individuals with unicoronal synostosis), strabismus, ptosis, and characteristic appearance of the ear (small pinna with a prominent superior and/or inferior crus). Syndactyly of digits two and three of the hand is variably present. Cognitive development is usually normal, although those with a large genomic deletion are at an increased risk for intellectual challenges. Less common manifestations of SCS include other skeletal findings (parietal foramina, vertebral segmentation defects, radioulnar synostosis, maxillary hypoplasia, ocular hypertelorism, hallux valgus, duplicated or curved distal hallux), hypertelorism, palatal anomalies, obstructive sleep apnea, increased intracranial pressure, short stature, and congenital heart malformations.

Aarskog-Scott syndrome is a genetic disorder that affects the development of many parts of the body, most commonly the head and face, the hands and feet, and the genitals and urinary system (genitourinary tract). This condition mainly affects males, although females may have mild features of the syndrome.\n\nPeople with Aarskog-Scott syndrome often have distinctive facial features, such as widely spaced eyes (hypertelorism), a small nose, a long area between the nose and mouth (philtrum), and a widow's peak hairline. They frequently have mild to moderate short stature during childhood, but their growth usually catches up with that of their peers during puberty. Hand abnormalities are common in this syndrome and include short fingers (brachydactyly), curved pinky fingers (fifth finger clinodactyly), webbing of the skin between some fingers (cutaneous syndactyly), and a single crease across the palm. Affected individuals can also have wide, flat feet with broad, rounded toes. Other abnormalities in people with Aarskog-Scott syndrome include heart defects and a split in the upper lip (cleft lip) with or without an opening in the roof of the mouth (cleft palate).\n\nMost males with Aarskog-Scott syndrome have a shawl scrotum, in which the scrotum surrounds the penis instead of hanging below. Less often, they have undescended testes (cryptorchidism) or a soft out-pouching around the belly-button (umbilical hernia) or in the lower abdomen (inguinal hernia).\n\nThe intellectual development of people with Aarskog-Scott syndrome varies widely. Most individuals with Aarskog-Scott syndrome have normal intelligence; however, some may have mild learning and behavior problems, and in rare cases, severe intellectual disability has been reported.

Williams syndrome (WS) is characterized by cardiovascular disease (elastin arteriopathy, peripheral pulmonary stenosis, supravalvar aortic stenosis, hypertension), distinctive facies, connective tissue abnormalities, intellectual disability (usually mild), a specific cognitive profile, unique personality characteristics, growth abnormalities, and endocrine abnormalities (hypercalcemia, hypercalciuria, hypothyroidism, and early puberty). Feeding difficulties often lead to poor weight gain in infancy. Hypotonia and hyperextensible joints can result in delayed attainment of motor milestones.

Thrombocytopenia absent radius (TAR) syndrome is characterized by bilateral absence of the radii with the presence of both thumbs, and thrombocytopenia that is generally transient. Thrombocytopenia may be congenital or may develop within the first few weeks to months of life; in general, thrombocytopenic episodes decrease with age. Cow's milk allergy is common and can be associated with exacerbation of thrombocytopenia. Other anomalies of the skeleton (upper and lower limbs, ribs, and vertebrae), heart, and genitourinary system (renal anomalies and agenesis of uterus, cervix, and upper part of the vagina) can occur.

Pfeiffer syndrome is an autosomal dominant craniosynostosis syndrome with characteristic anomalies of the hands and feet. Three clinical subtypes, which have important diagnostic and prognostic implications, have been identified. Type 1, the classic syndrome, is compatible with life and consists of craniosynostosis, midface deficiency, broad thumbs, broad great toes, brachydactyly, and variable syndactyly. Type 2 consists of cloverleaf skull with Pfeiffer hands and feet, together with ankylosis of the elbows. Type 3 is similar to type 2 but without cloverleaf skull. Ocular proptosis is severe, and the anterior cranial base is markedly short. Various visceral malformations have been found in association with type 3. Early demise is characteristic of types 2 and 3 (Cohen, 1993). Cohen and Barone (1994) further tabulated the findings in the 3 types of Pfeiffer syndrome.

Blepharophimosis, ptosis, and epicanthus inversus syndrome (BPES) is defined by a complex eyelid malformation characterized by four major features, all present at birth: blepharophimosis, ptosis, epicanthus inversus, and telecanthus. BPES type I includes the four major features and primary ovarian insufficiency; BPES type II includes only the four major features. Other ophthalmic manifestations that can be associated with BPES include dysplastic eyelids, lacrimal duct anomalies, strabismus, refractive errors, and amblyopia. Other craniofacial features may include a broad nasal bridge and low-set ears.

Mucolipidosis IV (MLIV) is an ultra-rare lysosomal storage disorder characterized by severe psychomotor delay, progressive visual impairment, and achlorhydria. Individuals with MLIV typically present by the end of the first year of life with delayed developmental milestones (due to a developmental brain abnormality) and impaired vision (resulting from a combination of corneal clouding and retinal degeneration). By adolescence, all individuals with MLIV have severe visual impairment. A neurodegenerative component of MLIV has become more widely appreciated, with the majority of individuals demonstrating progressive spastic quadriparesis and loss of psychomotor skills starting in the second decade of life. About 5% of individuals have atypical MLIV, manifesting with less severe psychomotor impairment, but still exhibiting progressive retinal degeneration and achlorhydria.

Treacher Collins syndrome (TCS) is characterized by bilateral and symmetric downslanting palpebral fissures, malar hypoplasia, micrognathia, and external ear abnormalities. Hypoplasia of the zygomatic bones and mandible can cause significant feeding and respiratory difficulties. About 40%-50% of individuals have conductive hearing loss attributed most commonly to malformation of the ossicles and hypoplasia of the middle ear cavities. Inner ear structures tend to be normal. Other, less common abnormalities include cleft palate and unilateral or bilateral choanal stenosis or atresia. Typically intellect is normal.

A rare autosomal recessive inherited syndrome. It is characterized by xeroderma pigmentosum, mental retardation, dwarfism, hypogonadism, and neurologic abnormalities.

EZH2-related overgrowth includes EZH2-related Weaver syndrome at one end of the spectrum and tall stature at the other. Although most individuals diagnosed with a heterozygous EZH2 pathogenic variant have been identified because of a clinical suspicion of Weaver syndrome, a minority have been identified through molecular genetic testing of family members of probands or individuals with overgrowth who did not have a clinical diagnosis of Weaver syndrome. Thus, the extent of the phenotypic spectrum associated with a heterozygous EZH2 pathogenic variant is not yet known. Weaver syndrome is characterized by tall stature, variable intellect (ranging from normal intellect to severe intellectual disability), characteristic facial appearance, and a range of associated clinical features including advanced bone age, poor coordination, soft doughy skin, camptodactyly of the fingers and/or toes, umbilical hernia, abnormal tone, and hoarse low cry in infancy. Brain MRI has identified abnormalities in a few individuals with EZH2-related overgrowth. Neuroblastoma occurs at a slightly increased frequency in individuals with a heterozygous EZH2 pathogenic variant but data are insufficient to determine absolute risk. There is currently no evidence that additional malignancies (including hematologic malignancies) occur with increased frequency.

Freeman-Sheldon syndrome (FSS), or DA2A, is phenotypically similar to DA1. In addition to contractures of the hands and feet, FSS is characterized by oropharyngeal abnormalities, scoliosis, and a distinctive face that includes a very small oral orifice (often only a few millimeters in diameter at birth), puckered lips, and an H-shaped dimple of the chin; hence, FSS has been called 'whistling face syndrome.' The limb phenotypes of DA1 and FSS may be so similar that they can only be distinguished by the differences in facial morphology (summary by Bamshad et al., 2009). For a general phenotypic description and a discussion of genetic heterogeneity of distal arthrogryposis, see DA1 (108120).

Nager syndrome is the prototype for a group of disorders collectively referred to as the acrofacial dysostoses (AFDs), which are characterized by malformation of the craniofacial skeleton and the limbs. The major facial features of Nager syndrome include downslanted palpebral fissures, midface retrusion, and micrognathia, the latter of which often requires the placement of a tracheostomy in early childhood. Limb defects typically involve the anterior (radial) elements of the upper limbs and manifest as small or absent thumbs, triphalangeal thumbs, radial hypoplasia or aplasia, and radioulnar synostosis. Phocomelia of the upper limbs and, occasionally, lower-limb defects have also been reported. The presence of anterior upper-limb defects and the typical lack of lower-limb involvement distinguishes Nager syndrome from Miller syndrome (263750), another rare AFD; however, distinguishing Nager syndrome from other AFDs, including Miller syndrome, can be challenging (summary by Bernier et al., 2012).

Mietens syndrome is a very rare syndrome consisting of corneal opacity, nystagmus, strabismus, flexion contracture of the elbows with dislocation of the head of the radius and abnormally short ulnae and radii.

Baller-Gerold syndrome (BGS) can be suspected at birth in an infant with craniosynostosis and upper limb abnormality. The coronal suture is most commonly affected; the metopic, lambdoid, and sagittal sutures may also be involved alone or in combination. Upper limb abnormality can include a combination of thumb hypo- or aplasia and radial hypo- or aplasia and may be asymmetric. Malformation or absence of carpal or metacarpal bones has also been described. Skin lesions may appear anytime within the first few years after birth, typically beginning with erythema of the face and extremities and evolving into poikiloderma. Slow growth is apparent in infancy with eventual height and length typically at 4 SD below the mean.

Adenylosuccinase deficiency is an autosomal recessive inborn error of metabolism caused by an enzymatic defect in de novo purine synthesis (DNPS) pathway. ADSL deficiency leads to the accumulation of toxic intermediates, including succinyladenosine (S-Ado) and succinylaminoimidazole carboxamide riboside (SAICAr) in body fluids. There are 3 major phenotypic forms of the disorder that correlate with different values of the S-Ado and SAICAr concentration ratios (S-Ado/SAICAr) in the cerebrospinal fluid. These include the most severe fatal neonatal encephalopathy (S-Ado/SAICAr ratio less than 1); childhood form (type I) with severe psychomotor retardation (S-Ado/SAICAr ratio close to 1), and a milder form (type II) with psychomotor retardation or hypotonia (S-Ado/SAICAr ratio greater than 2) (summary by Baresova et al., 2012).

Methemoglobinemia due to NADH-cytochrome b5 reductase deficiency is an autosomal recessive disorder characterized clinically by decreased oxygen carrying capacity of the blood, with resultant cyanosis and hypoxia (review by Percy and Lappin, 2008). There are 2 types of methemoglobin reductase deficiency. In type I, the defect affects the soluble form of the enzyme, is restricted to red blood cells, and causes well-tolerated methemoglobinemia. In type II, the defect affects both the soluble and microsomal forms of the enzyme and is thus generalized, affecting red cells, leukocytes, and all body tissues. Type II methemoglobinemia is associated with mental deficiency and other neurologic symptoms. The neurologic symptoms may be related to the major role played by the cytochrome b5 system in the desaturation of fatty acids (Vives-Corrons et al., 1978; Kaplan et al., 1979).

Chanarin-Dorfman syndrome (CDS) is a rare autosomal recessive nonlysosomal inborn error of neutral lipid metabolism. Patients present with a nonbullous erythrodermic form of ichthyosis (NCIE; see 242300) with variable involvement of other organs, such as liver, central nervous system, eyes, and ears. Intracellular triacylglycerol droplets are present in most tissues, and diagnosis can be confirmed by a simple blood smear, in which the characteristic lipid droplets are observed in the cytoplasm of granulocytes (summary by Lefevre et al., 2001). Another form of neutral lipid storage disease without ichthyosis but with myopathy (NLSDM; 610717) is caused by mutation in the PNPLA2 gene (609059).

Gaucher disease (GD) encompasses a continuum of clinical findings from a perinatal lethal disorder to an asymptomatic type. The identification of three major clinical types (1, 2, and 3) and two other subtypes (perinatal-lethal and cardiovascular) is useful in determining prognosis and management. GD type 1 is characterized by the presence of clinical or radiographic evidence of bone disease (osteopenia, focal lytic or sclerotic lesions, and osteonecrosis), hepatosplenomegaly, anemia and thrombocytopenia, lung disease, and the absence of primary central nervous system disease. GD types 2 and 3 are characterized by the presence of primary neurologic disease; in the past, they were distinguished by age of onset and rate of disease progression, but these distinctions are not absolute. Disease with onset before age two years, limited psychomotor development, and a rapidly progressive course with death by age two to four years is classified as GD type 2. Individuals with GD type 3 may have onset before age two years, but often have a more slowly progressive course, with survival into the third or fourth decade. The perinatal-lethal form is associated with ichthyosiform or collodion skin abnormalities or with nonimmune hydrops fetalis. The cardiovascular form is characterized by calcification of the aortic and mitral valves, mild splenomegaly, corneal opacities, and supranuclear ophthalmoplegia. Cardiopulmonary complications have been described with all the clinical subtypes, although varying in frequency and severity.

Gaucher disease (GD) encompasses a continuum of clinical findings from a perinatal lethal disorder to an asymptomatic type. The identification of three major clinical types (1, 2, and 3) and two other subtypes (perinatal-lethal and cardiovascular) is useful in determining prognosis and management. GD type 1 is characterized by the presence of clinical or radiographic evidence of bone disease (osteopenia, focal lytic or sclerotic lesions, and osteonecrosis), hepatosplenomegaly, anemia and thrombocytopenia, lung disease, and the absence of primary central nervous system disease. GD types 2 and 3 are characterized by the presence of primary neurologic disease; in the past, they were distinguished by age of onset and rate of disease progression, but these distinctions are not absolute. Disease with onset before age two years, limited psychomotor development, and a rapidly progressive course with death by age two to four years is classified as GD type 2. Individuals with GD type 3 may have onset before age two years, but often have a more slowly progressive course, with survival into the third or fourth decade. The perinatal-lethal form is associated with ichthyosiform or collodion skin abnormalities or with nonimmune hydrops fetalis. The cardiovascular form is characterized by calcification of the aortic and mitral valves, mild splenomegaly, corneal opacities, and supranuclear ophthalmoplegia. Cardiopulmonary complications have been described with all the clinical subtypes, although varying in frequency and severity.

ATP6V0A2-related cutis laxa is characterized by generalized cutis laxa, findings associated with generalized connective tissue disorder, developmental delays, and a variety of neurologic findings including abnormality on brain MRI. At birth, hypotonia, overfolded skin, and distinctive facial features are present and enlarged fontanelles are often observed. During childhood, the characteristic facial features and thick or coarse hair may become quite pronounced. The skin findings decrease with age, although easy bruising and Ehlers-Danlos-like scars have been described in some. In most (not all) affected individuals, cortical and cerebellar malformations are observed on brain MRI. Nearly all affected individuals have developmental delays, seizures, and neurologic regression.

NR0B1-related adrenal hypoplasia congenita includes both X-linked adrenal hypoplasia congenita (X-linked AHC) and Xp21 deletion (previously called complex glycerol kinase deficiency). X-linked AHC is characterized by primary adrenal insufficiency and/or hypogonadotropic hypogonadism (HH). Adrenal insufficiency is acute infantile onset (average age 3 weeks) in approximately 60% of affected males and childhood onset (ages 1-9 years) in approximately 40%. HH typically manifests in a male with adrenal insufficiency as delayed puberty (i.e., onset age >14 years) and less commonly as arrested puberty at about Tanner Stage 3. Rarely, X-linked AHC manifests initially in early adulthood as delayed-onset adrenal insufficiency, partial HH, and/or infertility. Heterozygous females very occasionally have manifestations of adrenal insufficiency or hypogonadotropic hypogonadism. Xp21 deletion includes deletion of NR0B1 (causing X-linked AHC) and GK (causing glycerol kinase deficiency), and in some cases deletion of DMD (causing Duchenne muscular dystrophy). Developmental delay has been reported in males with Xp21 deletion when the deletion extends proximally to include DMD or when larger deletions extend distally to include IL1RAPL1 and DMD.

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis is a progressive renal disorder characterized by excessive urinary Ca(2+) and Mg(2+) excretion. There is progressive loss of kidney function, and in about 50% of cases, the need for renal replacement therapy arises as early as the second decade of life (summary by Muller et al., 2006). Amelogenesis imperfecta may also be present in some patients (Bardet et al., 2016). A similar disorder with renal magnesium wasting, renal failure, and nephrocalcinosis (HOMG5; 248190) is caused by mutations in another tight-junction gene, CLDN19 (610036), and is distinguished by the association of severe ocular involvement. For a discussion of phenotypic and genetic heterogeneity of familial hypomagnesemia, see HOMG1 (602014).

Tyrosinase-positive oculocutaneous albinism (OCA, type II; OCA2) is an autosomal recessive disorder in which the biosynthesis of melanin pigment is reduced in skin, hair, and eyes. Although affected infants may appear at birth to have OCA type I, or complete absence of melanin pigment, most patients with OCA type II acquire small amounts of pigment with age. Individuals with OCA type II have the characteristic visual anomalies associated with albinism, including decreased acuity and nystagmus, which are usually less severe than in OCA type I (Lee et al., 1994; King et al., 2001). OCA type II has a highly variable phenotype. The hair of affected individuals may turn darker with age, and pigmented nevi or freckles may be seen. African and African American individuals may have yellow hair and blue-gray or hazel irides. One phenotypic variant, 'brown OCA,' has been described in African and African American populations and is characterized by light brown hair and skin color and gray to tan irides. The hair and irides may turn darker with time and the skin may tan with sun exposure; the ocular features of albinism are present in all variants (King et al., 2001). In addition, previous reports of so-called 'autosomal recessive ocular albinism,' (see, e.g., Witkop et al., 1978 and O'Donnell et al., 1978) with little or no obvious skin involvement, are now considered most likely to be part of the phenotypic spectrum of OCA1 or OCA2 (Lee et al., 1994; King et al., 2001).

A disorder that is characterized by increased glutathione concentration in the plasma and urine.

PLA2G6-associated neurodegeneration (PLAN) comprises a continuum of three phenotypes with overlapping clinical and radiologic features: Infantile neuroaxonal dystrophy (INAD). Atypical neuroaxonal dystrophy (atypical NAD). PLA2G6-related dystonia-parkinsonism. INAD usually begins between ages six months and three years with psychomotor regression or delay, hypotonia, and progressive spastic tetraparesis. Many affected children never learn to walk or lose the ability shortly after attaining it. Strabismus, nystagmus, and optic atrophy are common. Disease progression is rapid, resulting in severe spasticity, progressive cognitive decline, and visual impairment. Many affected children do not survive beyond their first decade. Atypical NAD shows more phenotypic variability than INAD. In general, onset is in early childhood but can be as late as the end of the second decade. The presenting signs may be gait instability, ataxia, or speech delay and autistic features, which are sometimes the only evidence of disease for a year or more. Strabismus, nystagmus, and optic atrophy are common. Neuropsychiatric disturbances including impulsivity, poor attention span, hyperactivity, and emotional lability are also common. The course is fairly stable during early childhood and resembles static encephalopathy but is followed by neurologic deterioration between ages seven and 12 years. PLA2G6-related dystonia-parkinsonism has a variable age of onset, but most individuals present in early adulthood with gait disturbance or neuropsychiatric changes. Affected individuals consistently develop dystonia and parkinsonism (which may be accompanied by rapid cognitive decline) in their late teens to early twenties. Dystonia is most common in the hands and feet but may be more generalized. The most common features of parkinsonism in these individuals are bradykinesia, resting tremor, rigidity, and postural instability.

Leucine-sensitive hypoglycemia (LIH) is a condition in which symptomatic hypoglycemia is provoked by protein meals or the amino acid leucine (summary by Magge et al., 2004).

Optic atrophy type 1 (OPA1, or Kjer type optic atrophy) is characterized by bilateral and symmetric optic nerve pallor associated with insidious decrease in visual acuity (usually between ages 4 and 6 years), visual field defects, and color vision defects. Visual impairment is usually moderate (6/10 to 2/10), but ranges from mild or even insignificant to severe (legal blindness with acuity <1/20). The visual field defect is typically centrocecal, central, or paracentral; it is often large in those with severe disease. The color vision defect is often described as acquired blue-yellow loss (tritanopia). Other findings can include auditory neuropathy resulting in sensorineural hearing loss that ranges from severe and congenital to subclinical (i.e., identified by specific audiologic testing only). Visual evoked potentials are typically absent or delayed; pattern electroretinogram shows an abnormal N95:P50 ratio. Tritanopia is the classic feature of color vision defect, but more diffuse nonspecific dyschromatopsia is not uncommon. Ophthalmoscopic examination discloses temporal or diffuse pallor of the optic discs, sometimes associated with optic disc excavation. The neuroretinal rim shows some pallor in most cases, sometimes associated with a temporal pigmentary gray crescent.

Reis-Bucklers corneal dystrophy (CDRB) is an autosomal dominant disorder of the superficial corneal stroma that manifests as recurrent corneal erosions in early childhood. Affected individuals develop corneal opacities that result in significant visual impairment. Microscopically, CDRB may be differentiated from other forms of corneal dystrophy by confluent opacities in the Bowman layer and subepithelium, which are the product of extracellular bodies that stain red with Masson trichrome stain and appear as crystalloid rod-shaped bodies on transmission electron microscopy (summary by Tanhehco et al., 2006).

Multiple synostoses syndrome is characterized by multiple joint fusions, usually commencing in the hands, conductive deafness, and characteristic facial features, including a broad, tubular-shaped nose and a thin upper vermilion. Other features include brachydactyly, hypoplastic or absent middle phalanges, radial head dislocation, and pectus carinatum (summary by Takahashi et al., 2001). Genetic Heterogeneity of Multiple Synostoses Syndrome Other forms of multiple synostoses syndrome include SYNS2 (610017), caused by mutation in the GDF5 gene (601146) on chromosome 20q11; SYNS3 (612961), caused by mutation in the FGF9 gene (600921) on chromosome 13q12; and SYNS4 (617898), caused by mutation in the GDF6 gene (601147) on chromosome 8q22.

Several additional types of this disorder have been proposed, each affecting one or a few families.\n\nResearchers have identified multiple types of oculocutaneous albinism, which are distinguished by their specific skin, hair, and eye color changes and by their genetic cause. Oculocutaneous albinism type 1 is characterized by white hair, very pale skin, and light-colored irises. Type 2 is typically less severe than type 1; the skin is usually a creamy white color and hair may be light yellow, blond, or light brown. Type 3 includes a form of albinism called rufous oculocutaneous albinism, which usually affects dark-skinned people. Affected individuals have reddish-brown skin, ginger or red hair, and hazel or brown irises. Type 3 is often associated with milder vision abnormalities than the other forms of oculocutaneous albinism. Type 4 has signs and symptoms similar to those seen with type 2.\n\nOculocutaneous albinism is a group of conditions that affect coloring (pigmentation) of the skin, hair, and eyes. Affected individuals typically have very fair skin and white or light-colored hair. Long-term sun exposure greatly increases the risk of skin damage and skin cancers, including an aggressive form of skin cancer called melanoma, in people with this condition. Oculocutaneous albinism also reduces pigmentation of the colored part of the eye (the iris) and the light-sensitive tissue at the back of the eye (the retina). People with this condition usually have vision problems such as reduced sharpness; rapid, involuntary eye movements (nystagmus); and increased sensitivity to light (photophobia).

3-Hydroxyisobutyryl-CoA hydrolase deficiency (HIBCHD) is an autosomal recessive inborn error of metabolism characterized by severely delayed psychomotor development, neurodegeneration, increased lactic acid, and brain lesions in the basal ganglia (summary by Ferdinandusse et al., 2013).

D-bifunctional protein deficiency is a disorder of peroxisomal fatty acid beta-oxidation. See also peroxisomal acyl-CoA oxidase deficiency (264470), caused by mutation in the ACOX1 gene (609751) on chromosome 17q25. The clinical manifestations of these 2 deficiencies are similar to those of disorders of peroxisomal assembly, including X-linked adrenoleukodystrophy (ALD; 300100), Zellweger cerebrohepatorenal syndrome (see 214100) and neonatal adrenoleukodystrophy (NALD; see 601539) (Watkins et al., 1995). DBP deficiency has been classified into 3 subtypes depending upon the deficient enzyme activity. Type I is a deficiency of both 2-enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase; type II is a deficiency of hydratase activity alone; and type III is a deficiency of dehydrogenase activity alone. Virtually all patients with types I, II, and III have a severe phenotype characterized by infantile-onset of hypotonia, seizures, and abnormal facial features, and most die before age 2 years. McMillan et al. (2012) proposed a type IV deficiency on the basis of less severe features; these patients have a phenotype reminiscent of Perrault syndrome (PRLTS1; 233400). Pierce et al. (2010) noted that Perrault syndrome and DBP deficiency overlap clinically and suggested that DBP deficiency may be underdiagnosed.

PAX6-related aniridia occurs either as an isolated ocular abnormality or as part of the Wilms tumor-aniridia-genital anomalies-retardation (WAGR) syndrome. Aniridia is a pan ocular disorder affecting the cornea, iris, intraocular pressure (resulting in glaucoma), lens (cataract and lens subluxation), fovea (foveal hypoplasia), and optic nerve (optic nerve coloboma and hypoplasia). Individuals with aniridia characteristically show nystagmus and impaired visual acuity (usually 20/100 - 20/200); however, milder forms of aniridia with subtle iris architecture changes, good vision, and normal foveal structure do occur. Other ocular involvement may include strabismus and occasionally microphthalmia. Although the severity of aniridia can vary between and within families, little variability is usually observed in the two eyes of an affected individual. WAGR syndrome. The risk for Wilms tumor is 42.5%-77%; of those who develop Wilms tumor, 90% do so by age four years and 98% by age seven years. Genital anomalies in males can include cryptorchidism and hypospadias (sometimes resulting in ambiguous genitalia), urethral strictures, ureteric abnormalities, and gonadoblastoma. While females typically have normal external genitalia, they may have uterine abnormalities and streak ovaries. Intellectual disability (defined as IQ <74) is observed in 70%; behavioral abnormalities include attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder, anxiety, depression, and obsessive-compulsive disorder. Other individuals with WAGR syndrome can have normal intellect without behavioral problems.

Anterior segment dysgeneses (ASGD or ASMD) are a heterogeneous group of developmental disorders affecting the anterior segment of the eye, including the cornea, iris, lens, trabecular meshwork, and Schlemm canal. The clinical features of ASGD include iris hypoplasia, an enlarged or reduced corneal diameter, corneal vascularization and opacity, posterior embryotoxon, corectopia, polycoria, an abnormal iridocorneal angle, ectopia lentis, and anterior synechiae between the iris and posterior corneal surface (summary by Cheong et al., 2016). Anterior segment dysgenesis is sometimes divided into subtypes including aniridia (see 106210), Axenfeld and Rieger anomalies, iridogoniodysgenesis, Peters anomaly, and posterior embryotoxon (Gould and John, 2002). Patients with ASGD5 have been reported with the Peters anomaly, Axenfeld anomaly, and Rieger anomaly subtypes. Peters anomaly consists of a central corneal leukoma, absence of the posterior corneal stroma and Descemet membrane, and a variable degree of iris and lenticular attachments to the central aspect of the posterior cornea (Peters, 1906). It occurs as an isolated ocular abnormality or in association with other ocular defects. In Axenfeld anomaly, strands of iris tissue attach to the Schwalbe line; in Rieger anomaly, in addition to the attachment of iris tissue to the Schwalbe line, there is clinically evident iris stromal atrophy with hole or pseudo-hole formation and corectopia (summary by Smith and Traboulsi, 2012).

The branchiooculofacial syndrome (BOFS) is characterized by: branchial (cervical or infra- or supra-auricular) skin defects that range from barely perceptible thin skin or hair patch to erythematous "hemangiomatous" lesions to large weeping erosions; ocular anomalies that can include microphthalmia, anophthalmia, coloboma, and nasolacrimal duct stenosis/atresia; and facial anomalies that can include ocular hypertelorism or telecanthus, broad nasal tip, upslanted palpebral fissures, cleft lip or prominent philtral pillars that give the appearance of a repaired cleft lip (formerly called "pseudocleft lip") with or without cleft palate, upper lip pits, and lower facial weakness (asymmetric crying face or partial 7th cranial nerve weakness). Malformed and prominent pinnae and hearing loss from inner ear and/or petrous bone anomalies are common. Intellect is usually normal.

Congenital myasthenic syndromes (CMS) are a group of inherited disorders affecting the neuromuscular junction (NMJ). Patients present clinically with onset of variable muscle weakness between infancy and adulthood. These disorders have been classified according to the location of the defect: presynaptic, synaptic, and postsynaptic. CMS6 is an autosomal recessive CMS resulting from a presynaptic defect; patients have onset of symptoms in infancy or early childhood and tend to have sudden apneic episodes. Treatment with acetylcholinesterase inhibitors may be beneficial (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

GAPO syndrome is the acronymic designation for a complex of growth retardation, alopecia, pseudoanodontia (failure of tooth eruption), and progressive optic atrophy (Tipton and Gorlin, 1984). Ilker et al. (1999) and Bayram et al. (2014) noted that optic atrophy is not a consistent feature of the disorder.

Fukuyama congenital muscular dystrophy (FCMD) is characterized by hypotonia, symmetric generalized muscle weakness, and CNS migration disturbances that result in changes consistent with cobblestone lissencephaly with cerebral and cerebellar cortical dysplasia. Mild, typical, and severe phenotypes are recognized. Onset typically occurs in early infancy with poor suck, weak cry, and floppiness. Affected individuals have contractures of the hips, knees, and interphalangeal joints. Later features include myopathic facial appearance, pseudohypertrophy of the calves and forearms, motor and speech delays, intellectual disability, seizures, ophthalmologic abnormalities including visual impairment and retinal dysplasia, and progressive cardiac involvement after age ten years. Swallowing disturbance occurs in individuals with severe FCMD and in individuals older than age ten years, leading to recurrent aspiration pneumonia and death.

Monosomy 18q is a partial deletion of the long arm of chromosome 18 characterized by highly variable phenotype, most commonly including hypotonia, developmental delay, short stature, growth hormone deficiency, hearing loss and external ear anomalies, intellectual disability, palatal defects, dysmorphic facial features, skeletal anomalies (foot deformities, tapering fingers, scoliosis) and mood disorders.

Mulibrey nanism (MUL) is a rare autosomal recessive growth disorder with prenatal onset, including occasional progressive cardiomyopathy, characteristic facial features, failure of sexual maturation, insulin resistance with type 2 diabetes, and an increased risk for Wilms tumor (summary by Hamalainen et al., 2006).

While the majority of individuals with Costello syndrome share characteristic findings affecting multiple organ systems, the phenotypic spectrum is wide, ranging from a milder or attenuated phenotype to a severe phenotype with early lethal complications. Costello syndrome is typically characterized by failure to thrive in infancy as a result of severe postnatal feeding difficulties; short stature; developmental delay or intellectual disability; coarse facial features (full lips, large mouth, full nasal tip); curly or sparse, fine hair; loose, soft skin with deep palmar and plantar creases; papillomata of the face and perianal region; diffuse hypotonia and joint laxity with ulnar deviation of the wrists and fingers; tight Achilles tendons; and cardiac involvement including: cardiac hypertrophy (usually typical hypertrophic cardiomyopathy), congenital heart defect (usually valvar pulmonic stenosis), and arrhythmia (usually supraventricular tachycardia, especially chaotic atrial rhythm/multifocal atrial tachycardia or ectopic atrial tachycardia). Relative or absolute macrocephaly is typical, and postnatal cerebellar overgrowth can result in the development of a Chiari I malformation with associated anomalies including hydrocephalus or syringomyelia. Individuals with Costello syndrome have an approximately 15% lifetime risk for malignant tumors including rhabdomyosarcoma and neuroblastoma in young children and transitional cell carcinoma of the bladder in adolescents and young adults.

Floating-Harbor syndrome (FHS) is characterized by typical craniofacial features; low birth weight, normal head circumference, and short stature; bone age delay that normalizes between ages six and 12 years; skeletal anomalies (brachydactyly, clubbing, clinodactyly, short thumbs, prominent joints, clavicular abnormalities); severe receptive and expressive language impairment; hypernasality and high-pitched voice; and intellectual disability that is typically mild to moderate. Difficulties with temperament and behavior that are present in many children tend to improve in adulthood. Other features can include hyperopia and/or strabismus, conductive hearing loss, seizures, gastroesophageal reflux, renal anomalies (e.g., hydronephrosis / renal pelviectasis, cysts, and/or agenesis), and genital anomalies (e.g., hypospadias and/or undescended testes).

Cockayne syndrome (referred to as CS in this GeneReview) spans a continuous phenotypic spectrum that includes: CS type I, the "classic" or "moderate" form; CS type II, a more severe form with symptoms present at birth; this form overlaps with cerebrooculofacioskeletal (COFS) syndrome; CS type III, a milder and later-onset form; COFS syndrome, a fetal form of CS. CS type I is characterized by normal prenatal growth with the onset of growth and developmental abnormalities in the first two years. By the time the disease has become fully manifest, height, weight, and head circumference are far below the fifth percentile. Progressive impairment of vision, hearing, and central and peripheral nervous system function leads to severe disability; death typically occurs in the first or second decade. CS type II is characterized by growth failure at birth, with little or no postnatal neurologic development. Congenital cataracts or other structural anomalies of the eye may be present. Affected children have early postnatal contractures of the spine (kyphosis, scoliosis) and joints. Death usually occurs by age five years. CS type III is a phenotype in which major clinical features associated with CS only become apparent after age two years; growth and/or cognition exceeds the expectations for CS type I. COFS syndrome is characterized by very severe prenatal developmental anomalies (arthrogryposis and microphthalmia).

Cockayne syndrome (referred to as CS in this GeneReview) spans a continuous phenotypic spectrum that includes: CS type I, the "classic" or "moderate" form; CS type II, a more severe form with symptoms present at birth; this form overlaps with cerebrooculofacioskeletal (COFS) syndrome; CS type III, a milder and later-onset form; COFS syndrome, a fetal form of CS. CS type I is characterized by normal prenatal growth with the onset of growth and developmental abnormalities in the first two years. By the time the disease has become fully manifest, height, weight, and head circumference are far below the fifth percentile. Progressive impairment of vision, hearing, and central and peripheral nervous system function leads to severe disability; death typically occurs in the first or second decade. CS type II is characterized by growth failure at birth, with little or no postnatal neurologic development. Congenital cataracts or other structural anomalies of the eye may be present. Affected children have early postnatal contractures of the spine (kyphosis, scoliosis) and joints. Death usually occurs by age five years. CS type III is a phenotype in which major clinical features associated with CS only become apparent after age two years; growth and/or cognition exceeds the expectations for CS type I. COFS syndrome is characterized by very severe prenatal developmental anomalies (arthrogryposis and microphthalmia).

Duane syndrome is a strabismus condition clinically characterized by congenital non-progressive limited horizontal eye movement accompanied by globe retraction which results in narrowing of the palpebral fissure. The lateral movement anomaly results from failure of the abducens nucleus and nerve (cranial nerve VI) to fully innervate the lateral rectus muscle; globe retraction occurs as a result of abnormal innervation of the lateral rectus muscle by the oculomotor nerve (cranial nerve III). At birth, affected infants have restricted ability to move the affected eye(s) outward (abduction) and/or inward (adduction), though the limitations may not be recognized in early infancy. In addition, the globe retracts into the orbit with attempted adduction, accompanied by narrowing of the palpebral fissure. Many individuals with Duane syndrome have strabismus in primary gaze but can use a compensatory head turn to align the eyes, and thus can preserve binocular vision and avoid diplopia. Individuals with Duane syndrome who lack binocular vision are at risk for amblyopia. The majority of affected individuals with Duane syndrome have isolated Duane syndrome (i.e., they do not have other detected congenital anomalies). Other individuals with Duane syndrome fall into well-defined syndromic diagnoses. However, many individuals with Duane syndrome have non-ocular findings that do not fit a known syndrome; these individuals are included as part of the discussion of nonsyndromic Duane syndrome.

For a general discussion of autosomal dominant spinocerebellar ataxia (SCA), see SCA1 (164400).

Recombinant chromosome 8 syndrome (Rec8 syndrome) is a chromosomal disorder found among individuals of Hispanic descent with ancestry from the San Luis Valley of southern Colorado and northern New Mexico. Affected individuals typically have impaired intellectual development, congenital heart defects, seizures, a characteristic facial appearance with hypertelorism, thin upper lip, anteverted nares, wide face, and abnormal hair whorl, and other manifestations (Sujansky et al., 1993, summary by Graw et al., 2000).

Kleefstra syndrome is characterized by intellectual disability, autistic-like features, childhood hypotonia, and distinctive facial features. The majority of individuals function in the moderate-to-severe spectrum of intellectual disability although a few individuals have mild delay and total IQ within low-normal range. While most have severe expressive speech delay with little speech development, general language development is usually at a higher level, making nonverbal communication possible. A complex pattern of other findings can also be observed; these include heart defects, renal/urologic defects, genital defects in males, severe respiratory infections, epilepsy / febrile seizures, psychiatric disorders, and extreme apathy or catatonic-like features after puberty.

Jacobsen syndrome (JBS) is a contiguous gene deletion syndrome with major clinical features of growth retardation, psychomotor retardation, trigonocephaly, divergent intermittent strabismus, epicanthus, telecanthus, broad nasal bridge, short nose with anteverted nostrils, carp-shaped upper lip, retrognathia, low-set dysmorphic ears, bilateral camptodactyly, hammertoes, and isoimmune thrombocytopenia (Fryns et al., 1986, Epstein, 1986).

Smith-Magenis syndrome (SMS) is characterized by distinctive physical features (particularly coarse facial features that progress with age), developmental delay, cognitive impairment, behavioral abnormalities, sleep disturbance, and childhood-onset abdominal obesity. Infants have feeding difficulties, failure to thrive, hypotonia, hyporeflexia, prolonged napping or need to be awakened for feeds, and generalized lethargy. The majority of individuals function in the mild-to-moderate range of intellectual disability. The behavioral phenotype, including significant sleep disturbance, stereotypies, and maladaptive and self-injurious behaviors, is generally not recognized until age 18 months or older and continues to change until adulthood. Sensory issues are frequently noted; these may include avoidant behavior, as well as repetitive seeking of textures, sounds, and experiences. Toileting difficulties are common. Significant anxiety is common as are problems with executive functioning, including inattention, distractibility, hyperactivity, and impulsivity. Maladaptive behaviors include frequent outbursts / temper tantrums, attention-seeking behaviors, opposition, aggression, and self-injurious behaviors including self-hitting, self-biting, skin picking, inserting foreign objects into body orifices (polyembolokoilamania), and yanking fingernails and/or toenails (onychotillomania). Among the stereotypic behaviors described, the spasmodic upper-body squeeze or "self-hug" seems to be highly associated with SMS. An underlying developmental asynchrony, specifically emotional maturity delayed beyond intellectual functioning, may also contribute to maladaptive behaviors in people with SMS.

L1 syndrome involves a phenotypic spectrum ranging from severe to mild and includes three clinical phenotypes: X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS). MASA (mental retardation [intellectual disability], aphasia [delayed speech], spastic paraplegia [shuffling gait], adducted thumbs) syndrome including X-linked complicated hereditary spastic paraplegia type 1. X-linked complicated corpus callosum agenesis. Males with HSAS are born with severe hydrocephalus, adducted thumbs, and spasticity; intellectual disability is severe. In less severely affected males, hydrocephalus may be subclinically present and documented only because of developmental delay; intellectual disability ranges from mild (IQ: 50-70) to moderate (IQ: 30-50). It is important to note that all phenotypes can be observed in affected individuals within the same family.

Gomez-Lopez-Hernandez syndrome (GLHS), also known as cerebellotrigeminal dermal dysplasia, is a rare neurocutaneous syndrome classically characterized by the triad of rhombencephalosynapsis, trigeminal anesthesia, often giving rise to corneal opacities, and bilateral parietal or parietooccipital alopecia. However, trigeminal anesthesia is an inconsistent finding (summary by Sukhudyan et al., 2010).

Kabuki syndrome (KS) is characterized by typical facial features (long palpebral fissures with eversion of the lateral third of the lower eyelid; arched and broad eyebrows; short columella with depressed nasal tip; large, prominent, or cupped ears), minor skeletal anomalies, persistence of fetal fingertip pads, mild-to-moderate intellectual disability, and postnatal growth deficiency. Other findings may include: congenital heart defects, genitourinary anomalies, cleft lip and/or palate, gastrointestinal anomalies including anal atresia, ptosis and strabismus, and widely spaced teeth and hypodontia. Functional differences can include: increased susceptibility to infections and autoimmune disorders, seizures, endocrinologic abnormalities (including isolated premature thelarche in females), feeding problems, and hearing loss.

Marden-Walker syndrome (MWKS) is characterized by psychomotor retardation, a mask-like face with blepharophimosis, micrognathia and a high-arched or cleft palate, low-set ears, kyphoscoliosis, and joint contractures. Other features may include Dandy-Walker malformation with hydrocephalus and vertebral abnormalities (summary by Schrander-Stumpel et al., 1993). There are 2 distal arthrogryposis syndromes with features overlapping those of Marden-Walker syndrome that are also caused by heterozygous mutation in PIEZO2: distal arthrogryposis type 3 (DA3, or Gordon syndrome; 114300) and distal arthrogryposis type 5 (DA5; 108145), which are distinguished by the presence of cleft palate and ocular abnormalities, respectively. McMillin et al. (2014) suggested that the 3 disorders may represent variable expressivity of the same condition.

Myhre syndrome is a connective tissue disorder with multisystem involvement, progressive and proliferative fibrosis that may occur spontaneously or following trauma or surgery, mild-to-moderate intellectual disability, and in some instances, autistic-like behaviors. Organ systems primarily involved include: cardiovascular (congenital heart defects, long- and short-segment stenosis of the aorta and peripheral arteries, pericardial effusion, constrictive pericarditis, restrictive cardiomyopathy, and hypertension); respiratory (choanal stenosis, laryngotracheal narrowing, obstructive airway disease, or restrictive pulmonary disease), gastrointestinal (pyloric stenosis, duodenal strictures, severe constipation); and skin (thickened particularly on the hands and extensor surfaces). Additional findings include distinctive craniofacial features and skeletal involvement (intrauterine growth restriction, short stature, limited joint range of motion). To date, 55 individuals with molecularly confirmed Myhre syndrome have been reported.

The C syndrome, also known as Opitz trigonocephaly syndrome, is a malformation syndrome characterized by trigonocephaly, severe mental retardation, hypotonia, variable cardiac defects, redundant skin, and dysmorphic facial features, including upslanted palpebral fissures, epicanthal folds, depressed nasal bridge, and low-set, posteriorly rotated ears (summary by Kaname et al., 2007). C syndrome shows phenotypic overlap with Bohring-Opitz syndrome, or C-like syndrome (605039), a disorder with more severe features than C syndrome, caused by heterozygous mutation in the ASXL1 gene (612990) on chromosome 20q11.

Other features occur in only one or a few types of oral-facial digital syndrome. These features help distinguish the different forms of the disorder. For example, the most common form of oral-facial-digital syndrome, type I, is associated with polycystic kidney disease. This kidney disease is characterized by the growth of fluid-filled sacs (cysts) that interfere with the kidneys' ability to filter waste products from the blood. Other forms of oral-facial-digital syndrome are characterized by neurological problems, particular changes in the structure of the brain, bone abnormalities, vision loss, and heart defects.\n\nAbnormalities of the digits can affect both the fingers and the toes in people with oral-facial-digital syndrome. These abnormalities include fusion of certain fingers or toes (syndactyly), digits that are shorter than usual (brachydactyly), or digits that are unusually curved (clinodactyly). The presence of extra digits (polydactyly) is also seen in most forms of oral-facial-digital syndrome.\n\nDistinctive facial features often associated with oral-facial-digital syndrome include a split in the lip (a cleft lip); a wide nose with a broad, flat nasal bridge; and widely spaced eyes (hypertelorism).\n\nAbnormalities of the oral cavity that occur in many types of oral-facial-digital syndrome include a split (cleft) in the tongue, a tongue with an unusual lobed shape, and the growth of noncancerous tumors or nodules on the tongue. Affected individuals may also have extra, missing, or defective teeth. Another common feature is an opening in the roof of the mouth (a cleft palate). Some people with oral-facial-digital syndrome have bands of extra tissue (called hyperplastic frenula) that abnormally attach the lip to the gums.\n\nThe signs and symptoms of oral-facial-digital syndrome vary widely. However, most forms of this disorder involve problems with development of the oral cavity, facial features, and digits. Most forms are also associated with brain abnormalities and some degree of intellectual disability.\n\nResearchers have identified at least 13 potential forms of oral-facial-digital syndrome. The different types are classified by their patterns of signs and symptoms. However, the features of the various types overlap significantly, and some types are not well defined. The classification system for oral-facial-digital syndrome continues to evolve as researchers find more affected individuals and learn more about this disorder.\n\nOral-facial-digital syndrome is actually a group of related conditions that affect the development of the oral cavity (the mouth and teeth), facial features, and digits (fingers and toes).

Syndrome with characteristics of highly arched palate with bifid tongue and bilateral supernumerary lower canines, hamartomatous tongue, multiple frenula, hypertelorism, telecanthus, strabismus, broad and/or bifid nasal tip, short stature, bifid hallux, forked metatarsal, poly and syndactyly, mild intellectual deficit and specific retinal abnormalities (bilateral optic disc coloboma and retinal dysplasia with partial detachment). Less than ten cases have been described in the literature. The causative gene has not yet been identified.

Primrose syndrome is characterized by macrocephaly, hypotonia, developmental delay, intellectual disability with expressive speech delay, behavioral issues, a recognizable facial phenotype, radiographic features, and altered glucose metabolism. Additional features seen in adults: sparse body hair, distal muscle wasting, and contractures. Characteristic craniofacial features include brachycephaly, high anterior hairline, deeply set eyes, ptosis, downslanted palpebral fissures, high palate with torus palatinus, broad jaw, and large ears with small or absent lobes. Radiographic features include calcification of the external ear cartilage, multiple Wormian bones, platybasia, bathrocephaly, slender bones with exaggerated metaphyseal flaring, mild epiphyseal dysplasia, and spondylar dysplasia. Additional features include hearing impairment, ocular anomalies, cryptorchidism, and nonspecific findings on brain MRI.

Proud syndrome is an X-linked developmental disorder characterized by agenesis of the corpus callosum, severe mental retardation, seizures, and spasticity. Males are severely affected, whereas females may be unaffected or have a milder phenotype (Proud et al., 1992). Proud syndrome is part of a phenotypic spectrum of disorders caused by mutation in the ARX gene comprising a nearly continuous series of developmental disorders ranging from lissencephaly (LISX2; 300215) to Proud syndrome to infantile spasms without brain malformations (DEE1; 308350) to syndromic (309510) and nonsyndromic (300419) mental retardation (Kato et al., 2004; Wallerstein et al., 2008).

Most characteristically, Aicardi-Goutières syndrome (AGS) manifests as an early-onset encephalopathy that usually, but not always, results in severe intellectual and physical disability. A subgroup of infants with AGS present at birth with abnormal neurologic findings, hepatosplenomegaly, elevated liver enzymes, and thrombocytopenia, a picture highly suggestive of congenital infection. Otherwise, most affected infants present at variable times after the first few weeks of life, frequently after a period of apparently normal development. Typically, they demonstrate the subacute onset of a severe encephalopathy characterized by extreme irritability, intermittent sterile pyrexias, loss of skills, and slowing of head growth. Over time, as many as 40% develop chilblain skin lesions on the fingers, toes, and ears. It is becoming apparent that atypical, sometimes milder, cases of AGS exist, and thus the true extent of the phenotype associated with pathogenic variants in the AGS-related genes is not yet known.

Renpenning syndrome is an X-linked syndromic intellectual developmental disorder with clinically recognizable features. Affected individuals have microcephaly, short stature, small testes, and dysmorphic facies, including tall narrow face, upslanting palpebral fissures, abnormal nasal configuration, cupped ears, and short philtrum. The nose may appear long or bulbous, with overhanging columella. Less consistent manifestations include ocular colobomas, cardiac malformations, cleft palate, and anal anomalies. Stevenson et al. (2005) proposed that the various X-linked mental retardation syndromes due to PQBP1 mutations be combined under the name of Renpenning syndrome.

The Richieri-Costa/Guion-Almeida syndrome is characterized by mild mental retardation, short stature, microbrachycephaly, ptosis, esotropia, cleft lip/palate (Richieri-Costa and Guion-Almeida, 1992).

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

A rare primary bone dysplasia disorder with characteristics of severe short stature, coarse facies, thoracolumbar kyphoscoliosis and enlarged joints with contractures. Psychomotor delay and intellectual disability may also be associated. Radiographic features include flat vertebral bodies, lacy ossification of the metaphyses of long bones and iliac crests, and marked sclerosis of the skull base.

SCAX1 is an X-linked recessive neurologic disorder characterized by hypotonia at birth, delayed motor development, gait ataxia, difficulty standing, dysarthria, and slow eye movements. Brain MRI shows cerebellar ataxia (summary by Bertini et al., 2000). Genetic Heterogeneity of X-linked Spinocerebellar Ataxia X-linked recessive spinocerebellar ataxia (SCAX) is a clinically and genetically heterogeneous disorder. See also SCAX2 (302600), SCAX3 (301790), SCAX4 (301840), and SCAX5 (300703). SCAX6 (301310) is caused by mutation in the ABCB7 gene (300135).

Kumar-type X-linked syndromic intellectual developmental disorder (MRXSK) is an X-linked recessive disorder that shows phenotypic variability and multisystem involvement apparent from birth or early infancy. Most affected individuals are male, although 1 severely affected girl with a de novo THOC2 mutation has been reported. At the severe end of the spectrum, affected individuals have hypotonia, neonatal difficulties, failure to thrive with poor overall growth, feeding difficulties, respiratory insufficiency, visual impairment, profoundly impaired intellectual development with poor or absent speech, and motor abnormalities, such as inability to walk and hyperkinetic movements. Less severely affected individuals have mildly to moderately impaired intellectual development and speech delay. Additional features include behavioral abnormalities, hearing or visual defects, seizures, dysmorphic facial features, and brain imaging abnormalities (Kumar et al., 2015; Kumar et al., 2018; Kumar et al., 2020).

CLCN4-related neurodevelopmental disorder (CLCN4-NDD), an X-linked disorder, is characterized in the 36 males reported to date by developmental delay or intellectual disability, behavioral/mental health issues (e.g., autism spectrum disorder, anxiety, hyperactivity, and bipolar disorder), epilepsy, and gastrointestinal dysfunction. The five heterozygous females with a de novo CLCN4 variant reported to date had findings very similar to those of affected males. Twenty-two of 25 heterozygous females identified in family studies following identification of an affected male were unaffected or had only mild specific learning difficulties and/or mental health concerns, whereas three were more severely affected.

Bohring-Opitz syndrome (BOS) is characterized by distinctive facial features and posture, growth failure, variable but usually severe intellectual disability, and variable anomalies. The facial features may include microcephaly or trigonocephaly / prominent (but not fused) metopic ridge, hypotonic facies with full cheeks, synophrys, glabellar and eyelid nevus flammeus (simplex), prominent globes, widely set eyes, palate anomalies, and micrognathia. The BOS posture, which is most striking in early childhood and often becomes less apparent with age, is characterized by flexion at the elbows with ulnar deviation and flexion of the wrists and metacarpophalangeal joints. Feeding difficulties in early childhood, including cyclic vomiting, have a significant impact on overall health; feeding tends to improve with age. Seizures are common and typically responsive to standard epileptic medications. Minor cardiac anomalies and transient bradycardia and apnea may be present. Affected individuals may experience recurrent infections, which also tend to improve with age. Isolated case reports suggest that individuals with BOS are at greater risk for Wilms tumor than the general population, but large-scale epidemiologic studies have not been conducted.

The term '3MC syndrome' encompasses 4 rare autosomal recessive disorders that were previously designated the Carnevale, Mingarelli, Malpuech, and Michels syndromes, respectively. The main features of these syndromes are facial dysmorphism that includes hypertelorism, blepharophimosis, blepharoptosis, and highly arched eyebrows, which are present in 70 to 95% of cases. Cleft lip and palate, postnatal growth deficiency, cognitive impairment, and hearing loss are also consistent findings, occurring in 40 to 68% of cases. Craniosynostosis, radioulnar synostosis, and genital and vesicorenal anomalies occur in 20 to 30% of cases. Rare features include anterior chamber defects, cardiac anomalies, caudal appendage, umbilical hernia (omphalocele), and diastasis recti (summary by Rooryck et al., 2011). For a discussion of genetic heterogeneity of 3MC syndrome, see 3MC1 (257920).

The core phenotype of Elsahy-Waters syndrome consists of brachycephaly, facial asymmetry, marked hypertelorism, proptosis, blepharochalasis, midface hypoplasia, broad nose with concave nasal ridge, and prognathism; radicular dentin dysplasia with consequent obliterated pulp chambers, apical translucent cysts, recurrent infections, and early loss of teeth; vertebral fusions, particularly at C2-C3; and moderate mental retardation. Skin wrinkling over the glabellar region seems common, and in males, hypospadias has always been present. Inter- and intrafamilial variability has been reported regarding the presence of vertebral fusions, hearing loss, and dentigerous cysts. Midface hypoplasia, facial asymmetry, progressive dental anomalies, and impaired cognitive development become more evident in adulthood (summary by Castori et al., 2010).

Duane syndrome is a strabismus condition clinically characterized by congenital non-progressive limited horizontal eye movement accompanied by globe retraction which results in narrowing of the palpebral fissure. The lateral movement anomaly results from failure of the abducens nucleus and nerve (cranial nerve VI) to fully innervate the lateral rectus muscle; globe retraction occurs as a result of abnormal innervation of the lateral rectus muscle by the oculomotor nerve (cranial nerve III). At birth, affected infants have restricted ability to move the affected eye(s) outward (abduction) and/or inward (adduction), though the limitations may not be recognized in early infancy. In addition, the globe retracts into the orbit with attempted adduction, accompanied by narrowing of the palpebral fissure. Many individuals with Duane syndrome have strabismus in primary gaze but can use a compensatory head turn to align the eyes, and thus can preserve binocular vision and avoid diplopia. Individuals with Duane syndrome who lack binocular vision are at risk for amblyopia. The majority of affected individuals with Duane syndrome have isolated Duane syndrome (i.e., they do not have other detected congenital anomalies). Other individuals with Duane syndrome fall into well-defined syndromic diagnoses. However, many individuals with Duane syndrome have non-ocular findings that do not fit a known syndrome; these individuals are included as part of the discussion of nonsyndromic Duane syndrome.

Dyskeratosis congenita and related telomere biology disorders (DC/TBD) are caused by impaired telomere maintenance resulting in short or very short telomeres. The phenotypic spectrum of telomere biology disorders is broad and includes individuals with classic dyskeratosis congenita (DC) as well as those with very short telomeres and an isolated physical finding. Classic DC is characterized by a triad of dysplastic nails, lacy reticular pigmentation of the upper chest and/or neck, and oral leukoplakia, although this may not be present in all individuals. People with DC/TBD are at increased risk for progressive bone marrow failure (BMF), myelodysplastic syndrome or acute myelogenous leukemia, solid tumors (usually squamous cell carcinoma of the head/neck or anogenital cancer), and pulmonary fibrosis. Other findings can include eye abnormalities (epiphora, blepharitis, sparse eyelashes, ectropion, entropion, trichiasis), taurodontism, liver disease, gastrointestinal telangiectasias, and avascular necrosis of the hips or shoulders. Although most persons with DC/TBD have normal psychomotor development and normal neurologic function, significant developmental delay is present in both forms; additional findings include cerebellar hypoplasia (Hoyeraal Hreidarsson syndrome) and bilateral exudative retinopathy and intracranial calcifications (Revesz syndrome and Coats plus syndrome). Onset and progression of manifestations of DC/TBD vary: at the mild end of the spectrum are those who have only minimal physical findings with normal bone marrow function, and at the severe end are those who have the diagnostic triad and early-onset BMF.

Cardiofaciocutaneous (CFC) syndrome is characterized by cardiac abnormalities (pulmonic stenosis and other valve dysplasias, septal defects, hypertrophic cardiomyopathy, rhythm disturbances), distinctive craniofacial appearance, and cutaneous abnormalities (including xerosis, hyperkeratosis, ichthyosis, keratosis pilaris, ulerythema ophryogenes, eczema, pigmented moles, hemangiomas, and palmoplantar hyperkeratosis). The hair is typically sparse, curly, fine or thick, woolly or brittle; eyelashes and eyebrows may be absent or sparse. Nails may be dystrophic or fast growing. Some form of neurologic and/or cognitive delay (ranging from mild to severe) is seen in all affected individuals. Neoplasia, mostly acute lymphoblastic leukemia, has been reported in some individuals.

Shprintzen-Goldberg syndrome (SGS) is characterized by: delayed motor and cognitive milestones and mild-to-moderate intellectual disability; craniosynostosis of the coronal, sagittal, or lambdoid sutures; distinctive craniofacial features; and musculoskeletal findings including olichostenomelia, arachnodactyly, camptodactyly, pectus excavatum or carinatum, scoliosis, joint hypermobility or contractures, pes planus, foot malposition, and C1-C2 spine malformation. Cardiovascular anomalies may include mitral valve prolapse, secundum atrial septal defect, and aortic root dilatation. Minimal subcutaneous fat, abdominal wall defects, and myopia are also characteristic findings.

IVIC syndrome (IVIC) is an autosomal dominant disorder characterized by upper limb anomalies (radial ray defects, carpal bone fusion), extraocular motor disturbances, and congenital bilateral nonprogressive mixed hearing loss. More variable features include heart involvement, mild thrombocytopenia and leukocytosis (before age 50), shoulder girdle hypoplasia, imperforate anus, kidney malrotation, and rectovaginal fistula (summary by Paradisi and Arias, 2007).

SALL4-related disorders include Duane-radial ray syndrome (DRRS, Okihiro syndrome), acro-renal-ocular syndrome (AROS), and SALL4-related Holt-Oram syndrome (HOS) – three phenotypes previously thought to be distinct entities. DRRS is characterized by uni- or bilateral Duane anomaly and radial ray malformation that can include thenar hypoplasia and/or hypoplasia or aplasia of the thumbs, hypoplasia or aplasia of the radii, shortening and radial deviation of the forearms, triphalangeal thumbs, and duplication of the thumb (preaxial polydactyly). AROS is characterized by radial ray malformations, renal abnormalities (mild malrotation, ectopia, horseshoe kidney, renal hypoplasia, vesicoureteral reflux, bladder diverticula), ocular coloboma, and Duane anomaly. Rarely, pathogenic variants in SALL4 may cause clinically typical HOS (i.e., radial ray malformations and cardiac malformations without additional features).

Groenouw type I, or granular type I, corneal dystrophy (CDGG1) is an autosomal dominant disorder characterized by irregular aggregates of hyaline material in the corneal stroma. These aggregates can cause significant visual disturbance and may require corneal transplantation for restoration of visual acuity or for relief from recurrent corneal erosions (summary by Stone et al., 1994).

Cayman cerebellar ataxia (ATCAY) is an autosomal recessive neurologic disorder characterized by hypotonia from birth, variable psychomotor retardation, and cerebellar dysfunction, including nystagmus, intention tremor, dysarthria, ataxic gait, and truncal ataxia. Although the disorder was initially believed to be restricted to an isolated region of Grand Cayman Island (summary by Nystuen et al., 1996; Bomar et al., 2003), one Pakistani family with the disorder and an ATCAY mutation has been reported, thus expanding the ethnic distribution (Manzoor et al., 2018).

Congenital disorders of glycosylation (CDGs) are a genetically heterogeneous group of autosomal recessive disorders caused by enzymatic defects in the synthesis and processing of asparagine (N)-linked glycans or oligosaccharides on glycoproteins. Type I CDGs comprise defects in the assembly of the dolichol lipid-linked oligosaccharide (LLO) chain and its transfer to the nascent protein. These disorders can be identified by a characteristic abnormal isoelectric focusing profile of plasma transferrin (Leroy, 2006). CDG1D is a type I CDG that generally presents with severe neurologic involvement associated with dysmorphism and visual impairment. Liver involvement is sometimes present (summary by Marques-da-Silva et al., 2017). For a discussion of the classification of CDGs, see CDG1A (212065).

Classic congenital or infantile nystagmus presents as conjugate, horizontal oscillations of the eyes, in primary or eccentric gaze, often with a preferred head turn or tilt. Other associated features may include mildly decreased visual acuity, strabismus, astigmatism, and occasionally head nodding. Eye movement recordings reveal that infantile nystagmus is predominantly a horizontal jerk waveform, with a diagnostic accelerating velocity slow phase. However, pendular and triangular waveforms may also be present. The nystagmus may rarely be vertical. As these patients often have normal visual acuity, it is presumed that the nystagmus represents a primary defect in the parts of the brain responsible for ocular motor control; thus the disorder has sometimes been termed 'congenital motor nystagmus' (Tarpey et al., 2006; Shiels et al., 2007). For a discussion of genetic heterogeneity of congenital nystagmus, see NYS1 (310700).

A very rare genetic overgrowth/obesity syndrome with characteristics of macrocephaly, obesity, mental (intellectual) disability and ocular abnormalities. Other frequent clinical signs include macrosomia, downslanting palpebral fissures, hypertelorism, broad nasal root, high and broad forehead and delay in bone maturation, in association with normal thyroid function and karyotype.

Cone dystrophy with supernormal rod responses (CDSRR) is characterized by onset in the first or second decade of life of very marked photophobia, myopia, reduced color vision along the red-green axis with relatively preserved tritan discrimination, and central scotomata with peripheral widespread sensitivity loss predominating in the superior visual field. Nyctalopia is a later feature of the disorder. There is often retinal pigment epithelium disturbance at the macula with a normal retinal periphery. Autofluorescence (AF) imaging shows either a perifoveal ring or a central macular area of relative increased AF (summary by Michaelides et al., 2005).

Disease with characteristics of early-onset tremor, dyskinesia and slowly progressive cerebellar ataxia. Fewer than 30 cases have been reported to date. This disease is caused by a mutation in the fibroblast growth factor 14 FGF14 gene (13q34). Prognosis is relatively good. Life-threatening status epilepticus and intractable seizure or severe dysphagia is rare.

Alpha-N-acetylgalactosaminidase (NAGA) deficiency is a very rare lysosomal storage disorder. It is clinically heterogeneous with 3 main phenotypes: type I is an infantile-onset neuroaxonal dystrophy; type II, also known as Kanzaki disease (609242), is an adult-onset disorder characterized by angiokeratoma corporis diffusum and mild intellectual impairment; and type III is an intermediate disorder with mild to moderate neurologic manifestations (Desnick and Schindler, 2001).

Congenital disorders of glycosylation (CDGs) are metabolic deficiencies in glycoprotein biosynthesis that usually cause severe mental and psychomotor retardation. Different forms of CDGs can be recognized by altered isoelectric focusing (IEF) patterns of serum transferrin. For a general discussion of CDGs, see CDG Ia (212065) and CDG Ib (602579).

A multiple congenital anomalies syndrome, described in one family to date, with characteristics of branchial cysts or fistula, ear malformations, congenital hearing loss (conductive, sensorineural, and mixed), internal auditory canal hypoplasia, strabismus, trismus, abnormal fifth fingers, vitiliginous lesions, short stature and mild learning disability. Renal and urethral abnormalities are absent.

Emanuel syndrome is characterized by pre- and postnatal growth deficiency, microcephaly, hypotonia, severe developmental delays, ear anomalies, preauricular tags or pits, cleft or high-arched palate, congenital heart defects, kidney abnormalities, and genital abnormalities in males.

Spondylometaphyseal dysplasia with cone-rod dystrophy (SMDCRD) is characterized by postnatal growth deficiency resulting in profound short stature, rhizomelia with bowing of the lower extremities, platyspondyly with anterior vertebral protrusions, progressive metaphyseal irregularity and cupping with shortened tubular bones, and early-onset progressive visual impairment associated with a pigmentary maculopathy and electroretinographic evidence of cone-rod dysfunction (summary by Hoover-Fong et al., 2014). Yamamoto et al. (2014) reviewed 16 reported cases of SMDCRD, noting that all affected individuals presented uniform skeletal findings, with rhizomelia and bowed lower limbs observed in the first year of life, whereas retinal dystrophy had a more variable age of onset. There was severe disproportionate short stature, with a final height of less than 100 cm; scoliosis was usually mild. Visual loss was progressive, with stabilization in adolescence.

Congenital myasthenic syndrome associated with AChR deficiency is a disorder of the postsynaptic neuromuscular junction (NMJ) clinically characterized by early-onset muscle weakness with variable severity. Electrophysiologic studies show low amplitude of the miniature endplate potential (MEPP) and current (MEPC) resulting from deficiency of AChR at the endplate. Patients with mutations in the CHRNE gene may have compensatory increased expression of the fetal subunit CHRNG (100730) and may respond to treatment with cholinergic agents, pyridostigmine, or amifampridine (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Congenital disorders of glycosylation (CDGs) are metabolic deficiencies in glycoprotein biosynthesis that usually cause severe mental and psychomotor retardation. Different forms of CDGs can be recognized by altered isoelectric focusing (IEF) patterns of serum transferrin. For a general discussion of CDGs, see CDG Ia (212065) and CDG Ib (602579).

Spinocerebellar ataxia-25 (SCA25) is an autosomal dominant neurologic disorder characterized by the onset of lower limb ataxia resulting in gait difficulties in the first few decades of life, although later onset has been reported. Affected individuals often have upper limb involvement, dysarthria, scoliosis, abnormal eye movements, and sensory neuropathy with decreased reflexes. Some patients have sensorineural hearing loss. Brain imaging shows cerebellar atrophy. There is incomplete penetrance and variable expressivity, even within families (Barbier et al., 2022). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Oculoectodermal syndrome (OES) is characterized by the association of epibulbar dermoids and aplasia cutis congenita. Affected individuals exhibit congenital scalp lesions which are atrophic, nonscarring, hairless regions that are often multiple and asymmetric in distribution, and may have associated hamartomas. Ectodermal changes include linear hyperpigmentation that may follow the lines of Blaschko and, rarely, epidermal nevus-like lesions. Epibulbar dermoids may be uni- or bilateral. Additional ocular anomalies such as skin tags of the upper eyelid and rarely optic nerve or retinal changes or microphthalmia can be present. Phenotypic expression is highly variable, and various other abnormalities have occasionally been reported, including growth failure, lymphedema, and cardiovascular defects, as well as neurodevelopmental symptoms such as developmental delay, epilepsy, learning difficulties, and behavioral abnormalities. Benign tumor-like lesions such as nonossifying fibromas of the long bones and giant cell granulomas of the jaws have repeatedly been observed and appear to be age-dependent, becoming a common manifestation in individuals aged 5 years or older (summary by Boppudi et al., 2016).

Isolated complex I deficiency is a rare inborn error of metabolism due to mutations in nuclear or mitochondrial genes encoding subunits or assembly factors of the human mitochondrial complex I (NADH: ubiquinone oxidoreductase) and is characterized by a wide range of manifestations including marked and often fatal lactic acidosis, cardiomyopathy, leukoencephalopathy, pure myopathy and hepatopathy with tubulopathy. Among the numerous clinical phenotypes observed are Leigh syndrome, Leber hereditary optic neuropathy and MELAS syndrome (see these terms).

Syndrome with the association of skeletal abnormalities, cutis laxa, craniostenosis, ambiguous genitalia, psychomotor retardation and facial abnormalities. So far, it has been described in two males (maternal first cousins). The mode of inheritance was suggested to be X-linked recessive.

Prieto syndrome (PRS) is an X-linked intellectual developmental disorder characterized by mildly to severely impaired intellectual development, developmental delay, autism spectrum disorder, or neuropsychiatric symptoms, variably accompanied by speech delay, epilepsy, microcephaly, structural brain defects, and minor facial anomalies (summary by Kury et al., 2022).

L1 syndrome involves a phenotypic spectrum ranging from severe to mild and includes three clinical phenotypes: X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS). MASA (mental retardation [intellectual disability], aphasia [delayed speech], spastic paraplegia [shuffling gait], adducted thumbs) syndrome including X-linked complicated hereditary spastic paraplegia type 1. X-linked complicated corpus callosum agenesis. Males with HSAS are born with severe hydrocephalus, adducted thumbs, and spasticity; intellectual disability is severe. In less severely affected males, hydrocephalus may be subclinically present and documented only because of developmental delay; intellectual disability ranges from mild (IQ: 50-70) to moderate (IQ: 30-50). It is important to note that all phenotypes can be observed in affected individuals within the same family.

A malformation syndrome involving the abnormal growth of the facial skeleton as well as its soft tissue structure and organs. The syndrome has characteristics of mild facial asymmetry with unaffected neurocranium and eyeballs, along with esotropia, amblyopia and/or convergent strabismus and occasionally submucous cleft palate. Transmission is autosomal dominant. There have been no further descriptions in the literature since 1979.

People with MCPH usually have few or no other features associated with the condition. Some have a narrow, sloping forehead; mild seizures; problems with attention or behavior; or short stature compared to others in their family. The condition typically does not affect any other major organ systems or cause other health problems.\n\nInfants with MCPH have an unusually small head circumference compared to other infants of the same sex and age. Head circumference is the distance around the widest part of the head, measured by placing a measuring tape above the eyebrows and ears and around the back of the head. Affected infants' brain volume is also smaller than usual, although they usually do not have any major abnormalities in the structure of the brain. The head and brain grow throughout childhood and adolescence, but they continue to be much smaller than normal.\n\nMCPH causes intellectual disability, which is typically mild to moderate and does not become more severe with age. Most affected individuals have delayed speech and language skills. Motor skills, such as sitting, standing, and walking, may also be mildly delayed.\n\nAutosomal recessive primary microcephaly (often shortened to MCPH, which stands for "microcephaly primary hereditary") is a condition in which infants are born with a very small head and a small brain. The term "microcephaly" comes from the Greek words for "small head."

Hermansky-Pudlak syndrome (HPS) is characterized by oculocutaneous albinism, a bleeding diathesis, and, in some individuals, pulmonary fibrosis, granulomatous colitis, or immunodeficiency. Ocular findings include reduced iris pigment with iris transillumination, reduced retinal pigment, foveal hypoplasia with significant reduction in visual acuity (usually in the range of 20/50 to 20/400), nystagmus, and increased crossing of the optic nerve fibers. Hair color ranges from white to brown; skin color ranges from white to olive and is usually a shade lighter than that of other family members. The bleeding diathesis can result in variable bruising, epistaxis, gingival bleeding, postpartum hemorrhage, colonic bleeding, and prolonged bleeding with menses or after tooth extraction, circumcision, and other surgeries. Pulmonary fibrosis, a restrictive lung disease, typically causes symptoms in the early thirties and can progress to death within a decade. Granulomatous colitis is severe in about 15% of affected individuals. Neutropenia and/or immune defects occur primarily in individuals with pathogenic variants in AP3B1 and AP3D1.

Gaucher disease (GD) encompasses a continuum of clinical findings from a perinatal lethal disorder to an asymptomatic type. The identification of three major clinical types (1, 2, and 3) and two other subtypes (perinatal-lethal and cardiovascular) is useful in determining prognosis and management. GD type 1 is characterized by the presence of clinical or radiographic evidence of bone disease (osteopenia, focal lytic or sclerotic lesions, and osteonecrosis), hepatosplenomegaly, anemia and thrombocytopenia, lung disease, and the absence of primary central nervous system disease. GD types 2 and 3 are characterized by the presence of primary neurologic disease; in the past, they were distinguished by age of onset and rate of disease progression, but these distinctions are not absolute. Disease with onset before age two years, limited psychomotor development, and a rapidly progressive course with death by age two to four years is classified as GD type 2. Individuals with GD type 3 may have onset before age two years, but often have a more slowly progressive course, with survival into the third or fourth decade. The perinatal-lethal form is associated with ichthyosiform or collodion skin abnormalities or with nonimmune hydrops fetalis. The cardiovascular form is characterized by calcification of the aortic and mitral valves, mild splenomegaly, corneal opacities, and supranuclear ophthalmoplegia. Cardiopulmonary complications have been described with all the clinical subtypes, although varying in frequency and severity.

The constitutional deletion of chromosome 1p36 results in a syndrome with multiple congenital anomalies and mental retardation (Shapira et al., 1997). Monosomy 1p36 is the most common terminal deletion syndrome in humans, occurring in 1 in 5,000 births (Shaffer and Lupski, 2000; Heilstedt et al., 2003). See also neurodevelopmental disorder with or without anomalies of the brain, eye, or heart (NEDBEH; 616975), which shows overlapping features and is caused by heterozygous mutation in the RERE gene (605226) on proximal chromosome 1p36. See also Radio-Tartaglia syndrome (RATARS; 619312), caused by mutation in the SPEN gene (613484) on chromosome 1p36, which shows overlapping features.

Spinocerebellar ataxia-21 (SCA21) is an autosomal dominant neurologic disorder characterized by onset in the first decades of life of slowly progressive cerebellar ataxia, which is associated with cognitive impairment in most patients (summary by Delplanque et al., 2014). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

X-linked spinocerebellar ataxia-6 with or without sideroblastic anemia (SCAX6) is an X-linked recessive disorder characterized by delayed motor development apparent in infancy with delayed walking (often by several years) due to ataxia and poor coordination. Additional features may include dysmetria, dysarthria, spasticity of the lower limbs, hyperreflexia, dysdiadochokinesis, strabismus, and nystagmus. The disorder is slowly progressive, and patients often lose ambulation. Brain imaging usually shows cerebellar atrophy. Most affected individuals have mild hypochromic, microcytic sideroblastic anemia, which may be asymptomatic. Laboratory studies show increased free erythrocyte protoporphyrin (FEP) and ringed sideroblasts on bone marrow biopsy. Female carriers do not have neurologic abnormalities, but may have subtle findings on peripheral blood smear (Pagon et al., 1985; D'Hooghe et al., 2012). For a discussion of genetic heterogeneity of X-linked spinocerebellar ataxia (SCAX), see SCAX1 (302500).

Classic congenital or infantile nystagmus presents as conjugate, horizontal oscillations of the eyes, in primary or eccentric gaze, often with a preferred head turn or tilt. Other associated features may include mildly decreased visual acuity, strabismus, astigmatism, and occasionally head nodding. Eye movement recordings reveal that infantile nystagmus is predominantly a horizontal jerk waveform, with a diagnostic accelerating velocity slow phase. However, pendular and triangular waveforms may also be present. The nystagmus may rarely be vertical. As these patients often have normal visual acuity, it is presumed that the nystagmus represents a primary defect in the parts of the brain responsible for ocular motor control; thus the disorder has sometimes been termed 'congenital motor nystagmus' (Tarpey et al., 2006; Shiels et al., 2007). For a discussion of genetic heterogeneity of congenital nystagmus, see NYS1 (310700).

The physical features of FG syndrome include weak muscle tone (hypotonia), broad thumbs, and wide first (big) toes. Abnormalities of the tissue connecting the left and right halves of the brain (the corpus callosum) are also common. Most affected individuals have constipation, and many have abnormalities of the anus such as an obstruction of the anal opening (imperforate anus). People with FG syndrome also tend to have a distinctive facial appearance including small, underdeveloped ears; a tall, prominent forehead; and outside corners of the eyes that point downward (down-slanting palpebral fissures).\n\nFG syndrome affects intelligence and behavior. Almost everyone with the condition has intellectual disability, which ranges from mild to severe. Affected individuals tend to be friendly, inquisitive, and hyperactive, with a short attention span. Compared to people with other forms of intellectual disability, their socialization and daily living skills are strong, while verbal communication and language skills tend to be weaker.\n\nFG syndrome is a genetic condition that affects many parts of the body and occurs almost exclusively in males. "FG" represents the surname initials of the first family diagnosed with the disorder.\n\nAdditional features seen in some people with FG syndrome include widely set eyes (hypertelorism), an upswept frontal hairline, and a large head compared to body size (relative macrocephaly). Other health problems have also been reported, including heart defects, seizures, undescended testes (cryptorchidism) in males, and a soft out-pouching in the lower abdomen (an inguinal hernia).

Claes-Jensen type of X-linked syndromic intellectual developmental disorder (MRXSCJ) is characterized by impaired intellectual development with substantial clinical heterogeneity in affected males. However, males are usually reported to have short stature, microcephaly, hyperreflexia, and aggressive behavior. In rare cases, female carriers exhibit mildly impaired intellectual development or learning difficulties (summary by Guerra et al., 2020).

X-linked intellectual deficit-cerebellar hypoplasia, also known as OPHN1 syndrome, is a rare syndromic form of cerebellar dysgenesis characterized by moderate to severe intellectual deficit and cerebellar abnormalities.

X-linked intellectual disability, Stocco Dos Santos type is characterised by severe intellectual deficit with hyperactivity, language delay, congenital hip luxation, short stature, kyphosis and recurrent respiratory infections. Aggressive behaviour and frequent epileptic seizures may also be present. The syndrome has been described in four boys from the same family. Transmission is X-linked and is caused by mutations in the <i>KIAA1202</i> gene, localised to the Xp11.2 region.

Spastic paraplegias (SPGs) are a genetically heterogeneous group of neurologic disorders characterized by progressive weakness and spasticity of the legs. Complicated SPGs are accompanied by additional neurologic symptoms such as cerebellar ataxia, sensory loss, mental retardation, nystagmus, and optic atrophy (summary by Steinmuller et al., 1997). A locus for spastic paraplegia-16 has been mapped to Xq11.2-q23 (Steinmuller et al., 1997). For a discussion of genetic heterogeneity of X-linked spastic paraplegia, see 303350.

MRXSA is an X-linked recessive neurodevelopmental disorder characterized by global developmental delay with impaired intellectual development, usually accompanied by walking difficulties and poor or absent speech. Affected individuals have dysmorphic features, including large head circumference, downslanting palpebral fissures, bulbous nose, high-arched palate, short stature, and small hands and feet. Ocular anomalies, including strabismus, exotropia, myopia, and keratoconus, are common. Some patients may develop seizures. Additional variable features, such as mild congenital heart defects, joint stiffness, renal anomalies, and hemangiomas, may also be present (summary by Lee et al., 2020).

MEHMO syndrome is a rare intellectual disability disorder that exhibits phenotypic heterogeneity and is variably characterized by mental retardation, epileptic seizures, hypogonadism with hypogenitalism, microcephaly, and obesity. Life expectancy ranges from less than 1 year to adulthood, and the condition is associated with significant morbidity and mortality (summary by Gregory et al., 2019).

FLNA deficiency is associated with a phenotypic spectrum that includes FLNA-related periventricular nodular heterotopia (Huttenlocher syndrome), congenital heart disease (patent ductus arteriosus, atrial and ventricular septal defects), valvular dystrophy, dilation and rupture of the thoracic aortic, pulmonary disease (pulmonary hypertension, alveolar hypoplasia, emphysema, asthma, chronic bronchitis), gastrointestinal dysmotility and obstruction, joint hypermobility, and macrothrombocytopenia.

CHIME syndrome, also known as Zunich neuroectodermal syndrome, is an extremely rare autosomal recessive multisystem disorder clinically characterized by colobomas, congenital heart defects, migratory ichthyosiform dermatosis, mental retardation, and ear anomalies (CHIME). Other clinical features include distinctive facial features, abnormal growth, genitourinary abnormalities, seizures, and feeding difficulties (summary by Ng et al., 2012). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Patients with mutations in the receptor for insulin-like growth factor I show intrauterine growth retardation and postnatal growth failure, resulting in short stature and microcephaly. Other features may include delayed bone age, developmental delay, and dysmorphic features.

A subtype of retinitis pigmentosa in which, instead of the pathology starting in the mid-periphery like typical retinitis pigmentosa, the disease starts in the near periphery closer to the vascular arcades and tends to spare the far periphery.

Pyridoxine-dependent epilepsy – ALDH7A1 (PDE-ALDH7A1) is characterized by seizures not well controlled with anti-seizure medication that are responsive clinically and electrographically to large daily supplements of pyridoxine (vitamin B6). This is true across a phenotypic spectrum that ranges from classic to atypical PDE-ALDH7A1. Intellectual disability is common, particularly in classic PDE-ALDH7A1. Classic PDE-ALDH7A1. Untreated seizures begin within the first weeks to months of life. Dramatic presentations of prolonged seizures and recurrent episodes of status epilepticus are typical; recurrent self-limited events including partial seizures, generalized seizures, atonic seizures, myoclonic events, and infantile spasms also occur. Electrographic seizures can occur without clinical correlates. Atypical PDE-ALDH7A1. Findings in untreated individuals can include late-onset seizures beginning between late infancy and age three years, seizures that initially respond to anti-seizure medication and then become intractable, seizures during early life that do not respond to pyridoxine but are subsequently controlled with pyridoxine several months later, and prolonged seizure-free intervals (=5 months) that occur after discontinuation of pyridoxine.

Peroxisomal acyl-CoA oxidase deficiency is a disorder of peroxisomal fatty acid beta-oxidation. See also D-bifunctional protein deficiency (261515), caused by mutation in the HSD17B4 gene (601860) on chromosome 5q2. The clinical manifestations of these 2 deficiencies are similar to those of disorders of peroxisomal assembly, including Zellweger cerebrohepatorenal syndrome (see 214100) and neonatal adrenoleukodystrophy (see 601539) (Watkins et al., 1995).

Total ophthalmoplegia involves total paralysis of all extra- and intraocular muscles. If one or more of the external muscles, including the levator palpebrae, is not affected, the condition is known as incomplete or partial ophthalmoplegia. If only 1 nerve is affected, the palsy is named after that nerve (see, e.g., abducens palsy, 100200) (Waardenburg, 1963).

GAN-related neurodegeneration comprises a phenotypic continuum ranging from severe (sometimes called classic giant axonal neuropathy) to milder pure early-onset peripheral motor and sensory neuropathies. The classic giant axonal neuropathy phenotype typically manifests as an infantile-onset neurodegenerative disorder, starting as a severe peripheral motor and sensory neuropathy and evolving into central nervous system impairment (intellectual disability, seizures, cerebellar signs, and pyramidal tract signs). Most affected individuals become wheelchair dependent in the second decade of life and eventually bedridden with severe polyneuropathy, ataxia, and dementia. Death usually occurs in the third decade. At the milder end of the spectrum are predominantly motor and sensory neuropathies (with little to no CNS involvement) that overlap with the axonal form of Charcot-Marie-Tooth neuropathies.

Keipert syndrome (KPTS) is characterized by craniofacial and digital abnormalities and variable learning difficulties. The distinctive facial appearance includes broad forehead, hypertelorism, prominent nose, wide mouth, and prominent upper lip with cupid bow configuration. Digital anomalies are also distinctive, with widening of all distal phalanges, particularly of the thumbs and great toes (Amor et al., 2019).

PAX2-related disorder is an autosomal dominant disorder associated with renal and eye abnormalities. The disorder was originally referred to as renal coloboma syndrome and characterized by renal hypodysplasia and abnormalities of the optic nerve; with improved access to molecular testing, a wider range of phenotypes has been recognized in association with pathogenic variants in PAX2. Abnormal renal structure or function is noted in 92% of affected individuals and ophthalmologic abnormalities in 77% of affected individuals. Renal abnormalities can be clinically silent in rare individuals. In most individuals, clinically significant renal insufficiency / renal failure is reported. End-stage renal disease requiring renal transplant is not uncommon. Uric acid nephrolithiasis has been reported. Ophthalmologic abnormalities are typically described as optic nerve coloboma or dysplasia. Iris colobomas have not been reported in any individual with PAX2–related disorder. Ophthalmologic abnormalities may significantly impair vision in some individuals, while others have subtle changes only noted after detailed ophthalmologic examination. Additional clinical findings include high-frequency sensorineural hearing loss, soft skin, and ligamentous laxity. PAX2 pathogenic variants have been identified in multiple sporadic and familial cases of nonsyndromic renal disease including renal hypodysplasia and focal segmental glomerulosclerosis.

SYNE1 deficiency comprises a phenotypic spectrum that ranges from autosomal recessive cerebellar ataxia at the mild end to arthrogryposis multiplex congenita (AMC) at the severe end. SYNE1-deficient cerebellar ataxia, the most commonly recognized manifestation of SYNE1 deficiency to date, is a slowly progressive disorder typically beginning in adulthood (age range 6-45 years). While some individuals have a pure cerebellar syndrome (i.e., cerebellar ataxia, dysarthria, dysmetria, abnormalities in ocular saccades and smooth pursuit), many also have upper motor neuron dysfunction (spasticity, hyperreflexia, Babinski sign) and/or lower motor neuron dysfunction (amyotrophy, reduced reflexes, fasciculations). Most individuals develop features of the cerebellar cognitive and affective syndrome (i.e., significant deficits in attention, executive functioning, verbal working memory, and visuospatial/visuoconstructional skills). The two less common phenotypes are SYNE1-deficient childhood-onset multisystem disease (ataxia, upper and lower motor neuron dysfunction, muscle weakness and wasting, intellectual disability) and SYNE1-deficient arthrogryposis multiplex congenita (decreased fetal movements and severe neonatal hypotonia associated with multiple congenital joint contractures including clubfoot).

Phelan-McDermid syndrome is characterized by neonatal hypotonia, absent to severely delayed speech, developmental delay, and minor dysmorphic facial features. Most affected individuals have moderate to profound intellectual disability. Other features include large fleshy hands, dysplastic toenails, and decreased perspiration that results in a tendency to overheat. Normal stature and normal head size distinguishes Phelan-McDermid syndrome from other autosomal chromosome disorders. Behavior characteristics include mouthing or chewing non-food items, decreased perception of pain, and autism spectrum disorder or autistic-like affect and behavior.

Intellectual developmental disorder with short stature, facial anomalies, and speech defects (IDDSFAS) is an autosomal recessive disorder characterized by stature below the 1st centile, mild facial dysmorphism comprising mainly large bulbous nose and microretrognathia, and developmental delay with varying degrees of intellectual disability (Ansar et al., 2019).

Ataxia with oculomotor apraxia type 2 (AOA2) is characterized by onset of ataxia between age three and 30 years after initial normal development, axonal sensorimotor neuropathy, oculomotor apraxia, cerebellar atrophy, and elevated serum concentration of alpha-fetoprotein (AFP).

Wiedemann-Steiner syndrome (WSS) is characterized by developmental delay, intellectual disability, and characteristic facial features, with or without additional congenital anomalies. The facial features include thick eyebrows with lateral flare, vertically narrow and downslanted palpebral fissures, widely spaced eyes, long eyelashes, wide nasal bridge, broad nasal tip, thin vermilion of the upper lip, and thick scalp hair. About 60% of affected individuals have hypertrichosis cubiti ("hairy elbows"), which was once thought to be pathognomic for the syndrome, with a majority having hypertrichosis of other body parts. Other clinical features include feeding difficulties, prenatal and postnatal growth restriction, epilepsy, ophthalmologic anomalies, congenital heart defects, hand anomalies (such as brachydactyly and clinodactyly), hypotonia, vertebral anomalies (especially fusion anomalies of the cervical spine), renal and uterine anomalies, immune dysfunction, brain malformations, and dental anomalies.

Metaphyseal dysostosis-intellectual disability-conductive deafness syndrome is characterised by metaphyseal dysplasia, short-limb dwarfism, mild intellectual deficit and conductive hearing loss, associated with repeated episodes of otitis media in childhood. It has been described in three brothers born to consanguineous Sicilian parents. Variable manifestations included hyperopia and strabismus. The mode of inheritance is autosomal recessive.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the PRSS12 gene.

Kaufman oculocerebrofacial syndrome (KOS) is characterized by developmental delay, severe intellectual disability, and distinctive craniofacial features. Most affected children have prenatal-onset microcephaly, hypotonia, and growth deficiency. Feeding issues, ocular abnormalities, hearing impairment, and respiratory tract abnormalities are common. Ocular abnormalities can include structural abnormalities (microcornea or microphthalmia, coloboma, optic nerve hypoplasia), refractive errors (myopia ± astigmatism, hyperopia), strabismus, and entropion. Both conductive and sensorineural hearing loss have been reported as well as mixed conductive-sensorineural hearing loss of variable severity. Breathing problems can lead to prolonged hospitalization after birth in more than half of individuals. Less common findings include ectodermal abnormalities, cardiac manifestations, urogenital abnormalities, seizures, and skeletal abnormalities.

2-hydroxyglutaric aciduria is a condition that causes progressive damage to the brain. The major types of this disorder are called D-2-hydroxyglutaric aciduria (D-2-HGA), L-2-hydroxyglutaric aciduria (L-2-HGA), and combined D,L-2-hydroxyglutaric aciduria (D,L-2-HGA).\n\nThe main features of D-2-HGA are delayed development, seizures, weak muscle tone (hypotonia), and abnormalities in the largest part of the brain (the cerebrum), which controls many important functions such as muscle movement, speech, vision, thinking, emotion, and memory. Researchers have described two subtypes of D-2-HGA, type I and type II. The two subtypes are distinguished by their genetic cause and pattern of inheritance, although they also have some differences in signs and symptoms. Type II tends to begin earlier and often causes more severe health problems than type I. Type II may also be associated with a weakened and enlarged heart (cardiomyopathy), a feature that is typically not found with type I.\n\nL-2-HGA particularly affects a region of the brain called the cerebellum, which is involved in coordinating movements. As a result, many affected individuals have problems with balance and muscle coordination (ataxia). Additional features of L-2-HGA can include delayed development, seizures, speech difficulties, and an unusually large head (macrocephaly). Typically, signs and symptoms of this disorder begin during infancy or early childhood. The disorder worsens over time, usually leading to severe disability by early adulthood.\n\nCombined D,L-2-HGA causes severe brain abnormalities that become apparent in early infancy. Affected infants have severe seizures, weak muscle tone (hypotonia), and breathing and feeding problems. They usually survive only into infancy or early childhood.

Mowat-Wilson syndrome (MWS) is characterized by distinctive facial features (widely spaced eyes, broad eyebrows with a medial flare, low-hanging columella, prominent or pointed chin, open-mouth expression, and uplifted earlobes with a central depression), congenital heart defects with predilection for abnormalities of the pulmonary arteries and/or valves, Hirschsprung disease or chronic constipation, genitourinary anomalies (particularly hypospadias in males), and hypogenesis or agenesis of the corpus callosum. Most affected individuals have moderate-to-severe intellectual disability. Speech is typically limited to a few words or is absent, with relative preservation of receptive language skills. Growth restriction with microcephaly and seizure disorder are also common. Most affected people have a happy demeanor and a wide-based gait that can sometimes be confused with Angelman syndrome.

Gaucher disease (GD) encompasses a continuum of clinical findings from a perinatal lethal disorder to an asymptomatic type. The identification of three major clinical types (1, 2, and 3) and two other subtypes (perinatal-lethal and cardiovascular) is useful in determining prognosis and management. GD type 1 is characterized by the presence of clinical or radiographic evidence of bone disease (osteopenia, focal lytic or sclerotic lesions, and osteonecrosis), hepatosplenomegaly, anemia and thrombocytopenia, lung disease, and the absence of primary central nervous system disease. GD types 2 and 3 are characterized by the presence of primary neurologic disease; in the past, they were distinguished by age of onset and rate of disease progression, but these distinctions are not absolute. Disease with onset before age two years, limited psychomotor development, and a rapidly progressive course with death by age two to four years is classified as GD type 2. Individuals with GD type 3 may have onset before age two years, but often have a more slowly progressive course, with survival into the third or fourth decade. The perinatal-lethal form is associated with ichthyosiform or collodion skin abnormalities or with nonimmune hydrops fetalis. The cardiovascular form is characterized by calcification of the aortic and mitral valves, mild splenomegaly, corneal opacities, and supranuclear ophthalmoplegia. Cardiopulmonary complications have been described with all the clinical subtypes, although varying in frequency and severity.

Mitochondrial complex IV deficiency nuclear type 5 (MC4DN5) is an autosomal recessive severe metabolic multisystemic disorder with onset in infancy. Features include delayed psychomotor development, impaired intellectual development with speech delay, mild dysmorphic facial features, hypotonia, ataxia, and seizures. There is increased serum lactate and episodic hypoglycemia. Some patients may have cardiomyopathy, abnormal breathing, or liver abnormalities, reflecting systemic involvement. Brain imaging shows lesions in the brainstem and basal ganglia, consistent with a diagnosis of Leigh syndrome (see 256000). Affected individuals tend to have episodic metabolic and/or neurologic crises in early childhood, which often lead to early death (summary by Debray et al., 2011). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Mutations in the CRYAA gene have been found to cause multiple types of cataract, which have been described as nuclear, zonular central nuclear, laminar, lamellar, anterior polar, posterior polar, cortical, embryonal, anterior subcapsular, fan-shaped, and total. Cataract associated with microcornea, sometimes called the cataract-microcornea syndrome, is also caused by mutation in the CRYAA gene. Both autosomal dominant and autosomal recessive modes of inheritance have been reported. The symbol CATC1 was formerly used for the autosomal recessive form of cataract caused by mutation in the CRYAA gene.

This syndrome has characteristics of intellectual deficit, calcification of the choroid plexus and elevated levels of cerebrospinal fluid protein. It has been described in two sibships from two unrelated families. The seven children of one of the sibships were born to consanguineous parents. Some patients also had strabismus, hyperactive deep tendon reflexes and foot deformities.

Sengers syndrome is an autosomal recessive mitochondrial disorder characterized by congenital cataracts, hypertrophic cardiomyopathy, skeletal myopathy, exercise intolerance, and lactic acidosis. Mental development is normal, but affected individuals may die early from cardiomyopathy (summary by Mayr et al., 2012). Skeletal muscle biopsies of 2 affected individuals showed severe mtDNA depletion (Calvo et al., 2012).

BBS9 is an autosomal recessive disorder characterized by obesity, polydactyly, renal anomalies, retinopathy, and mental retardation (Abu-Safieh et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Strabismus, a misalignment of the eyes also referred to as 'squint,' is one of the most common ocular disorders in humans, affecting 1 to 4% of the population. It is frequently associated with amblyopia (uniocular visual neglect) (Parikh et al., 2003). Strabismus is also a feature of several syndromes, including congenital fibrosis of extraocular muscles (see, e.g., CFEOM1; 135700), Duane retraction syndrome (126800), and chronic progressive external ophthalmoplegia with myopathy (530000).

Spinocerebellar ataxia-29 (SCA29) is an autosomal dominant neurologic disorder characterized by onset in infancy of delayed motor development and mild cognitive delay. Affected individuals develop a very slowly progressive or nonprogressive gait and limb ataxia associated with cerebellar atrophy on brain imaging. Additional variable features include nystagmus, dysarthria, and tremor (summary by Huang et al., 2012). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

A rare multiple congenital anomalies/dysmorphic syndrome characterized by brachydactyly, nystagmus, and cerebellar ataxia. Intellectual deficit and strabismus have also been reported. There have been no further descriptions in the literature since 1934.

Distal arthrogryposis type 5 is distinguished from other forms of DA by the presence of ocular abnormalities, typically ptosis, ophthalmoplegia, and/or strabismus, in addition to contractures of the skeletal muscles. Some cases have been reported to have pulmonary hypertension as a result of restrictive lung disease (summary by Bamshad et al., 2009). There are 2 syndromes with features overlapping those of DA5 that are also caused by heterozygous mutation in PIEZO2: distal arthrogryposis type 3 (DA3, or Gordon syndrome; 114300) and Marden-Walker syndrome (MWKS; 248700), which are distinguished by the presence of cleft palate and mental retardation, respectively. McMillin et al. (2014) suggested that the 3 disorders might represent variable expressivity of the same condition. For a general phenotypic description and a discussion of genetic heterogeneity of distal arthrogryposis, see DA1A (108120). Genetic Heterogeneity of Distal Arthrogryposis 5 A subtype of DA5 due to mutation in the ECEL1 gene (605896) on chromosome 2q36 has been designated DA5D (615065). See NOMENCLATURE.

Muenke syndrome is defined by the presence of the specific FGFR3 pathogenic variant – c.749C>G – that results in the protein change p.Pro250Arg. Muenke syndrome is characterized by considerable phenotypic variability: features may include coronal synostosis (more often bilateral than unilateral); synostosis of other sutures, all sutures (pan synostosis), or no sutures; or macrocephaly. Bilateral coronal synostosis typically results in brachycephaly (reduced anteroposterior dimension of the skull), although turribrachycephaly (a "tower-shaped" skull) or a cloverleaf skull can be observed. Unilateral coronal synostosis results in anterior plagiocephaly (asymmetry of the skull and face). Other craniofacial findings typically include: temporal bossing; widely spaced eyes, ptosis or proptosis (usually mild); midface retrusion (usually mild); and highly arched palate or cleft lip and palate. Strabismus is common. Other findings can include: hearing loss (in 33%-100% of affected individuals); developmental delay (~33%); epilepsy; intracranial anomalies; intellectual disability; carpal bone and/or tarsal bone fusions; brachydactyly, broad toes, broad thumbs, and/or clinodactyly; and radiographic findings of thimble-like (short and broad) middle phalanges and/or cone-shaped epiphyses. Phenotypic variability is considerable even within the same family. Of note, some individuals who have the p.Pro250Arg pathogenic variant may have no signs of Muenke syndrome on physical or radiographic examination.

Chromosome 16p13.3deletion syndrome is a chromosome abnormality that can affect many parts of the body. People with this condition are missing a small piece (deletion) of chromosome 16 at a location designated p13.3. Although once thought to be a severe form of Rubinstein-Taybi syndrome, it is now emerging as a unique syndrome. Signs and symptoms may include failure to thrive, hypotonia (reduced muscle tone), short stature, microcephaly (unusually small head), characteristic facial features, mild to moderate intellectual disability, organ anomalies (i.e. heart and/or kidney problems), and vulnerability to infections. Chromosome testing of both parents can provide information about whether the deletion was inherited. In most cases, parents do not have any chromosome abnormalities. However, sometimes one parent has a balanced translocation where a piece of a chromosome has broken off and attached to another one with no gain or loss of genetic material. The balanced translocation normally does not cause signs or symptoms, but it increases the risk for having a child with a chromosome abnormality like a deletion. Treatment is based on the signs and symptoms present in each person.To learn more about chromosome abnormalities in general, view our GARD fact sheet on Chromosome Disorders.

Congenital stromal corneal dystrophy is characterized by the presence of bilateral corneal opacities that can be seen at or shortly after birth. The surface of the cornea is normal or slightly irregular; small opacities are seen throughout the stroma of the entire cornea and give the cornea a cloudy appearance. Strabismus is common. Nystagmus is uncommon. Amblyopia can develop in children.

Koolen-de Vries syndrome (KdVS) is characterized by developmental delay / intellectual disability, neonatal/childhood hypotonia, dysmorphisms, congenital malformations, and behavioral features. Psychomotor developmental delay is noted in all individuals from an early age. The majority of individuals with KdVS function in the mild-to-moderate range of intellectual disability. Other findings include speech and language delay (100%), epilepsy (~33%), congenital heart defects (25%-50%), renal and urologic anomalies (25%-50%), and cryptorchidism (71% of males). Behavior in most is described as friendly, amiable, and cooperative.

Spondyloepiphyseal dysplasia of the Genevieve type (SEMDG) is characterized by infantile-onset severe developmental delay and skeletal dysplasia, including short stature, premature carpal ossification, platyspondyly, longitudinal metaphyseal striations, and small epiphyses (summary by van Karnebeek et al., 2016).

Colobomatous macrophthalmia with microcornea (MACOM) is an autosomal dominant eye malformation characterized by microcornea with increased axial length, coloboma of the iris and optic disc, and severe myopia (summary by Beleggia et al., 2015).

Desmosterolosis is a rare autosomal recessive disorder characterized by multiple congenital anomalies and elevated levels of the cholesterol precursor desmosterol in plasma, tissue, and cultured cells (summary by Waterham et al., 2001).

Pierpont syndrome (PRPTS) is a multiple congenital anomaly syndrome associated with learning disability. Key features include distinctive facial characteristics, especially when smiling, plantar fat pads, and other limb anomalies (summary by Burkitt Wright et al., 2011).

Bleeding problems are common in the vascular type of Ehlers-Danlos syndrome and are caused by unpredictable tearing (rupture) of blood vessels and organs. These complications can lead to easy bruising, internal bleeding, a hole in the wall of the intestine (intestinal perforation), or stroke. During pregnancy, women with vascular Ehlers-Danlos syndrome may experience rupture of the uterus. Additional forms of Ehlers-Danlos syndrome that involve rupture of the blood vessels include the kyphoscoliotic, classical, and classical-like types.\n\nMany people with the Ehlers-Danlos syndromes have soft, velvety skin that is highly stretchy (elastic) and fragile. Affected individuals tend to bruise easily, and some types of the condition also cause abnormal scarring. People with the classical form of Ehlers-Danlos syndrome experience wounds that split open with little bleeding and leave scars that widen over time to create characteristic "cigarette paper" scars. The dermatosparaxis type of the disorder is characterized by loose skin that sags and wrinkles, and extra (redundant) folds of skin may be present.\n\nAn unusually large range of joint movement (hypermobility) occurs in most forms of Ehlers-Danlos syndrome, and it is a hallmark feature of the hypermobile type. Infants and children with hypermobility often have weak muscle tone (hypotonia), which can delay the development of motor skills such as sitting, standing, and walking. The loose joints are unstable and prone to dislocation and chronic pain. In the arthrochalasia type of Ehlers-Danlos syndrome, infants have hypermobility and dislocations of both hips at birth.\n\nThe various forms of Ehlers-Danlos syndrome have been classified in several different ways. Originally, 11 forms of Ehlers-Danlos syndrome were named using Roman numerals to indicate the types (type I, type II, and so on). In 1997, researchers proposed a simpler classification (the Villefranche nomenclature) that reduced the number of types to six and gave them descriptive names based on their major features. In 2017, the classification was updated to include rare forms of Ehlers-Danlos syndrome that were identified more recently. The 2017 classification describes 13 types of Ehlers-Danlos syndrome.\n\nOther types of Ehlers-Danlos syndrome have additional signs and symptoms. The cardiac-valvular type causes severe problems with the valves that control the movement of blood through the heart. People with the kyphoscoliotic type experience severe curvature of the spine that worsens over time and can interfere with breathing by restricting lung expansion. A type of Ehlers-Danlos syndrome called brittle cornea syndrome is characterized by thinness of the clear covering of the eye (the cornea) and other eye abnormalities. The spondylodysplastic type features short stature and skeletal abnormalities such as abnormally curved (bowed) limbs. Abnormalities of muscles, including hypotonia and permanently bent joints (contractures), are among the characteristic signs of the musculocontractural and myopathic forms of Ehlers-Danlos syndrome. The periodontal type causes abnormalities of the teeth and gums.\n\nEhlers-Danlos syndrome is a group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Defects in connective tissues cause the signs and symptoms of these conditions, which range from mildly loose joints to life-threatening complications.

About half of all people with trichothiodystrophy have a photosensitive form of the disorder, which causes them to be extremely sensitive to ultraviolet (UV) rays from sunlight. They develop a severe sunburn after spending just a few minutes in the sun. However, for reasons that are unclear, they do not develop other sun-related problems such as excessive freckling of the skin or an increased risk of skin cancer. Many people with trichothiodystrophy report that they do not sweat.\n\nMothers of children with trichothiodystrophy may experience problems during pregnancy including pregnancy-induced high blood pressure (preeclampsia) and a related condition called HELLP syndrome that can damage the liver. Babies with trichothiodystrophy are at increased risk of premature birth, low birth weight, and slow growth. Most children with trichothiodystrophy have short stature compared to others their age. \n\nThe signs and symptoms of trichothiodystrophy vary widely. Mild cases may involve only the hair. More severe cases also cause delayed development, significant intellectual disability, and recurrent infections; severely affected individuals may survive only into infancy or early childhood.\n\nIntellectual disability and delayed development are common in people with trichothiodystrophy, although most affected individuals are highly social with an outgoing and engaging personality. Some people with trichothiodystrophy have brain abnormalities that can be seen with imaging tests. A common neurological feature of this disorder is impaired myelin production (dysmyelination). Myelin is a fatty substance that insulates nerve cells and promotes the rapid transmission of nerve impulses.\n\nTrichothiodystrophy is also associated with recurrent infections, particularly respiratory infections, which can be life-threatening. People with trichothiodystrophy may have abnormal red blood cells, including red blood cells that are smaller than normal. They may also have elevated levels of a type of hemoglobin called A2, which is a protein found in red blood cells. Other features of trichothiodystrophy can include dry, scaly skin (ichthyosis); abnormalities of the fingernails and toenails; clouding of the lens in both eyes from birth (congenital cataracts); poor coordination; and skeletal abnormalities including degeneration of both hips at an early age.\n\nIn people with trichothiodystrophy, tests show that the hair is lacking sulfur-containing proteins that normally gives hair its strength. A cross section of a cut hair shows alternating light and dark banding that has been described as a "tiger tail."\n\nTrichothiodystrophy, commonly called TTD, is a rare inherited condition that affects many parts of the body. The hallmark of this condition is hair that is sparse and easily broken. 

A rare syndrome with characteristics of split-hand and split-foot deformity and ocular abnormalities mainly a congenital nystagmus. Ten cases from four families have been reported in the literature. In some cases the hands are monodactylous. The affected patients have normal mental development. The condition seems to be autosomal dominant with a relatively high proportion of gonadal mosaicism.

This syndrome is extremely rare and is characterized by delayed speech development, mild facial asymmetry, strabismus and transverse ear lobe creases.

Robinow-Sorauf syndrome is a condition with features similar to those of Saethre-Chotzen syndrome, including craniosynostosis and broad or duplicated great toes. It was once considered a separate disorder, but was found to result from mutations in the same gene and is now thought to be a variant of Saethre-Chotzen syndrome.\n\nThe signs and symptoms of Saethre-Chotzen syndrome vary widely, even among affected individuals in the same family. This condition can cause mild changes in the hands and feet, such as partial fusion of the skin between the second and third fingers on each hand and a broad or duplicated first (big) toe. Delayed development and learning difficulties have been reported, although most people with this condition are of normal intelligence. Less common signs and symptoms of Saethre-Chotzen syndrome include short stature, abnormalities of the bones of the spine (the vertebra), hearing loss, and heart defects.\n\nMost people with Saethre-Chotzen syndrome have prematurely fused skull bones along the coronal suture, the growth line that goes over the head from ear to ear. Other parts of the skull may be malformed as well. These changes can result in an abnormally shaped head, a high forehead, a low frontal hairline, droopy eyelids (ptosis), widely spaced eyes, and a broad nasal bridge. One side of the face may appear noticeably different from the other (facial asymmetry). Most people with Saethre-Chotzen syndrome also have small, rounded ears.\n\nSaethre-Chotzen syndrome is a genetic condition characterized by the premature fusion of certain skull bones (craniosynostosis). This early fusion prevents the skull from growing normally and affects the shape of the head and face.

A rare disorder characterized by the association of ptosis, strabismus and ectopic pupils. It has been described in one family (in a mother and three of her children). Transmission is autosomal dominant.

Char syndrome is characterized by the triad of typical facial features, patent ductus arteriosus, and aplasia or hypoplasia of the middle phalanges of the fifth fingers. Typical facial features are depressed nasal bridge and broad flat nasal tip, widely spaced eyes, downslanted palpebral fissures, mild ptosis, short philtrum with prominent philtral ridges with an upward pointing vermilion border resulting in a triangular mouth, and thickened (patulous) everted lips. Less common findings include other types of congenital heart defects, other hand and foot anomalies, hypodontia, hearing loss, myopia and/or strabismus, polythelia, parasomnia, craniosynostosis (involving either the metopic or sagittal suture), and short stature.

Wolf-Hirschhorn syndrome is a congenital malformation syndrome characterized by pre- and postnatal growth deficiency, developmental disability of variable degree, characteristic craniofacial features ('Greek warrior helmet' appearance of the nose, high forehead, prominent glabella, hypertelorism, high-arched eyebrows, protruding eyes, epicanthal folds, short philtrum, distinct mouth with downturned corners, and micrognathia), and a seizure disorder (Battaglia et al., 2008).

Alagille syndrome (ALGS) is a multisystem disorder with a wide spectrum of clinical variability; this variability is seen even among individuals from the same family. The major clinical manifestations of ALGS are bile duct paucity on liver biopsy, cholestasis, congenital cardiac defects (primarily involving the pulmonary arteries), butterfly vertebrae, ophthalmologic abnormalities (most commonly posterior embryotoxon), and characteristic facial features. Renal abnormalities, growth failure, developmental delays, splenomegaly, and vascular abnormalities may also occur.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the NSUN2 gene.

A rare genetic neurological disorder with characteristics of pregnancy complicated by polyhydramnios, severe intractable epilepsy presenting in infancy, severe hypotonia, decreased muscle mass, global developmental delay, craniofacial dysmorphism (long face, large forehead, peaked eyebrows, broad nasal bridge, hypertelorism, large mouth with thick lips), and macrocephaly due to megalencephaly and hydrocephalus in most patients. Additional features that have been reported include cardiac anomalies like atrial septal defects, diabetes insipidus and nephrocalcinosis among others.

Pitt-Hopkins syndrome (PTHS) is characterized by significant developmental delays with moderate-to-severe intellectual disability and behavioral differences, characteristic facial features, and episodic hyperventilation and/or breath-holding while awake. Speech is significantly delayed and most individuals are nonverbal with receptive language often stronger than expressive language. Other common findings are autism spectrum disorder symptoms, sleep disturbance, stereotypic hand movements, seizures, constipation, and severe myopia.

Phosphoribosylpyrophosphate synthetase (PRS) superactivity comprises two phenotypes, both characterized by hyperuricemia and hyperuricosuria. The mild phenotype (~75% of affected males) with onset in the second or third decade of life is typically limited to these biochemical findings, whereas the severe phenotype (~25% of affected males) with onset in the first decade of life has in addition to these biochemical findings variable combinations of developmental delay (DD) / intellectual disability (ID), sensorineural hearing loss, hypotonia, and ataxia. In the mild phenotype, uric acid crystalluria or a urinary stone is commonly the first clinical finding, followed later by gouty arthritis if serum urate concentration is not controlled.

10q26 deletion syndrome is a condition that results from the loss (deletion) of a small piece of chromosome 10 in each cell. The deletion occurs on the long (q) arm of the chromosome at a position designated 10q26.\n\nThe signs and symptoms of 10q26 deletion syndrome vary widely, even among affected members of the same family. Among the more common features associated with this chromosomal change are distinctive facial features, mild to moderate intellectual disability, growth problems, and developmental delay. People with 10q26 deletion syndrome often have delayed development of speech and of motor skills such as sitting, crawling, and walking. Some have limited speech throughout life. Affected individuals may experience seizures, attention-deficit/hyperactivity disorder (ADHD), poor impulse control (impulsivity), or exhibit autistic behaviors that affect communication and social interaction.\n\nA range of facial features is seen in people with 10q26 deletion syndrome, but not all affected individuals have these features. Facial features of people with 10q26 deletion syndrome may include a prominent or beaked nose, a broad nasal bridge, a small jaw (micrognathia), malformed ears that are low set, a thin upper lip, and an unusually small head size (microcephaly). Many affected individuals have widely spaced eyes (hypertelorism) that do not look in the same direction (strabismus). Some people with this condition have a short neck with extra folds of skin (webbed neck).\n\nLess common signs and symptoms can occur in 10q26 deletion syndrome. Skeletal problems include a spine that curves to the side (scoliosis), limited movement in the elbows or other joints, or curved fifth fingers and toes (clinodactyly). Slow growth before and after birth can also occur in affected individuals. Males with this condition may have genital abnormalities, such as a small penis (micropenis), undescended testes (cryptorchidism), or the urethra opening on the underside of the penis (hypospadias). Some people with 10q26 deletion syndrome have kidney abnormalities, heart defects, breathing problems, recurrent infections, or hearing or vision problems.

Distal monosomy 15q is a rare chromosomal anomaly syndrome characterized by pre- and postnatal growth restriction, developmental delay, variable degrees of intellectual disability, hand and foot anomalies (e.g. brachy-/clinodactyly, talipes equinovarus, nail hypoplasia, proximally placed digits) and mild craniofacial dysmorphism (incl. microcephaly, triangular face, broad nasal bridge, micrognathia). Neonatal lymphedema, heart malformations, aplasia cutis congenita, aortic root dilatation, and autistic spectrum disorder have also been reported.

SYNGAP1-related intellectual disability (SYNGAP1-ID) is characterized by developmental delay (DD) or intellectual disability (ID) (100% of affected individuals), generalized epilepsy (~84%), and autism spectrum disorder (ASD) and other behavioral abnormalities (=50%). To date more than 50 individuals with SYNGAP1-ID have been reported. In the majority DD/ID was moderate to severe; in some it was mild. The epilepsy is generalized; a subset of individuals with epilepsy have myoclonic astatic epilepsy (Doose syndrome) or epilepsy with myoclonic absences. Behavioral abnormalities can include stereotypic behaviors (e.g., hand flapping, obsessions with certain objects) as well as poor social development. Feeding difficulties can be significant in some.

Distal monosomy 6p is responsible for a distinct chromosome deletion syndrome with a recognizable clinical picture including intellectual deficit, ocular abnormalities, hearing loss, and facial dysmorphism.

Chromosome 2p16.1-p15 deletion syndrome is a neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability, and variable but distinctive dysmorphic features, including microcephaly, bitemporal narrowing, smooth and long philtrum, hypertelorism, downslanting palpebral fissures, broad nasal root, thin upper lip, and high palate. Many patients have behavioral disorders, including autistic features, as well as structural brain abnormalities, such as pachygyria or hypoplastic corpus callosum. Those with deletions including the BCL11A gene (606557) also have persistence of fetal hemoglobin (HbF), which is asymptomatic and does not affected hematologic parameters or susceptibility to infection (summary by Funnell et al., 2015). Point mutation in the BCL11A gene causes intellectual developmental disorder with persistence of fetal hemoglobin (617101), which shows overlapping features. See also fetal hemoglobin quantitative trait locus-5 (HBFQTL5; 142335).

The 1q21.1 recurrent microdeletion itself does not appear to lead to a clinically recognizable syndrome as some persons with the deletion have no obvious clinical findings and others have variable findings that most commonly include microcephaly (50%), mild intellectual disability (30%), mildly dysmorphic facial features, and eye abnormalities (26%). Other findings can include cardiac defects, genitourinary anomalies, skeletal malformations, and seizures (~15%). Psychiatric and behavioral abnormalities can include autism spectrum disorders, attention deficit hyperactivity disorder, autistic features, and sleep disturbances.

Diamond-Blackfan anemia (DBA) is characterized by a profound normochromic and usually macrocytic anemia with normal leukocytes and platelets, congenital malformations in up to 50%, and growth deficiency in 30% of affected individuals. The hematologic complications occur in 90% of affected individuals during the first year of life. The phenotypic spectrum ranges from a mild form (e.g., mild anemia or no anemia with only subtle erythroid abnormalities, physical malformations without anemia) to a severe form of fetal anemia resulting in nonimmune hydrops fetalis. DBA is associated with an increased risk for acute myelogenous leukemia (AML), myelodysplastic syndrome (MDS), and solid tumors including osteogenic sarcoma.

Any leukodystrophy in which the cause of the disease is a mutation in the HSPD1 gene.

Primary coenzyme Q10 (CoQ10) deficiency is usually associated with multisystem involvement, including neurologic manifestations such as fatal neonatal encephalopathy with hypotonia; a late-onset slowly progressive multiple-system atrophy-like phenotype (neurodegeneration with autonomic failure and various combinations of parkinsonism and cerebellar ataxia, and pyramidal dysfunction); and dystonia, spasticity, seizures, and intellectual disability. Steroid-resistant nephrotic syndrome (SRNS), the hallmark renal manifestation, is often the initial manifestation either as isolated renal involvement that progresses to end-stage renal disease (ESRD), or associated with encephalopathy (seizures, stroke-like episodes, severe neurologic impairment) resulting in early death. Hypertrophic cardiomyopathy (HCM), retinopathy or optic atrophy, and sensorineural hearing loss can also be seen.

Individuals with the 15q13.3 recurrent deletion may have a wide range of clinical manifestations. The deletion itself may not lead to a clinically recognizable syndrome and a subset of persons with the recurrent deletion have no obvious clinical findings, implying that penetrance for the deletion is incomplete. A little over half of individuals diagnosed with this recurrent deletion have intellectual disability or developmental delay, mainly in the areas of speech acquisition and cognitive function. In the majority of individuals, cognitive impairment is mild. Other features reported in diagnosed individuals include epilepsy (in ~30%), mild hypotonia, and neuropsychiatric disorders (including autism spectrum disorder, attention-deficit/hyperactivity disorder, mood disorder, schizophrenia, and aggressive or self-injurious behavior). Congenital malformations are uncommon.

CASK disorders include a spectrum of phenotypes in both females and males. Two main types of clinical presentation are seen: Microcephaly with pontine and cerebellar hypoplasia (MICPCH), generally associated with pathogenic loss-of-function variants in CASK. X-linked intellectual disability (XLID) with or without nystagmus, generally associated with hypomorphic CASK pathogenic variants. MICPCH is typically seen in females with moderate-to-severe intellectual disability, progressive microcephaly with or without ophthalmologic anomalies, and sensorineural hearing loss. Most are able to sit independently; 20%-25% attain the ability to walk; language is nearly absent in most. Neurologic features may include axial hypotonia, hypertonia/spasticity of the extremities, and dystonia or other movement disorders. Nearly 40% have seizures by age ten years. Behaviors may include sleep disturbances, hand stereotypies, and self biting. MICPCH in males may occur with or without severe epileptic encephalopathy in addition to severe-to-profound developmental delay. When seizures are present they occur early and may be intractable. In individuals and families with milder (i.e., hypomorphic) pathogenic variants, the clinical phenotype is usually that of XLID with or without nystagmus and additional clinical features. Males have mild-to-severe intellectual disability, with or without nystagmus and other ocular features. Females typically have normal intelligence with some displaying mild-to-severe intellectual disability with or without ocular features.

Turner-type X-linked syndromic intellectual developmental disorder (MRXST) is a neurodevelopmental disorder with a highly variable phenotype. Some affected families show X-linked recessive inheritance, with only males being affected and carrier females having no abnormal findings. In other affected families, males are severely affected, and female mutation carriers show milder cognitive abnormalities or dysmorphic features. In addition, there are female patients with de novo mutations who show the full phenotype, despite skewed X-chromosome inactivation. Affected individuals show global developmental delay from infancy, with variably impaired intellectual development and poor or absent speech, often with delayed walking. Dysmorphic features are common and can include macrocephaly, microcephaly, deep-set eyes, hypotelorism, small palpebral fissures, dysplastic, large, or low-set ears, long face, bitemporal narrowing, high-arched palate, thin upper lip, and scoliosis or mild distal skeletal anomalies, such as brachydactyly or tapered fingers. Males tend to have cryptorchidism. Other features, such as hypotonia, seizures, and delayed bone age, are more variable (summary by Moortgat et al., 2018).

Christianson syndrome (referred to as CS in this GeneReview), an X-linked disorder, is characterized in males by cognitive dysfunction, behavioral disorder, and neurologic findings (e.g., seizures, ataxia, postnatal microcephaly, and eye movement abnormalities). Males with CS typically present with developmental delay, later meeting criteria for severe intellectual disability (ID). Behaviorally, autism spectrum disorder and hyperactivity are common, and may resemble the behaviors observed in Angelman syndrome. Hypotonia and oropharyngeal dysphagia in infancy may result in failure to thrive. Seizures, typically beginning before age three years, can include infantile spasms and tonic, tonic-clonic, myoclonic, and atonic seizures. Subsequently, regression (e.g., loss of ambulation and ability to feed independently) may occur. Manifestations in heterozygous females range from asymptomatic to mild ID and/or behavioral issues.

SUCLA2-related mitochondrial DNA (mtDNA) depletion syndrome, encephalomyopathic form with methylmalonic aciduria is characterized by onset of the following features in infancy or childhood (median age of onset 2 months; range of onset birth to 6 years): psychomotor retardation, hypotonia, dystonia, muscular atrophy, sensorineural hearing impairment, postnatal growth retardation, and feeding difficulties. Other less frequent features include distinctive facial features, contractures, kyphoscoliosis, gastroesophageal reflux, ptosis, choreoathetosis, ophthalmoplegia, and epilepsy (infantile spasms or generalized convulsions). The median survival is 20 years; approximately 30% of affected individuals succumb during childhood. Affected individuals may have hyperintensities in the basal ganglia, cerebral atrophy, and leukoencephalopathy on head MRI. Elevation of methylmalonic acid (MMA) in the urine and plasma is found in a vast majority of affected individuals, although at levels that are far below those typically seen in individuals with classic methylmalonic aciduria.

Spastic paraplegia-18B (SPG18B) is a severe autosomal recessive neurologic disorder characterized by onset in early childhood of progressive spastic paraplegia resulting in motor disability. Most affected individuals have severe psychomotor retardation. Some may develop significant joint contractures (summary by Alazami et al., 2011 and Yildirim et al., 2011).

Cerebellar ataxia, impaired intellectual development, and dysequilibrium syndrome (CAMRQ) is a genetically heterogeneous disorder characterized by congenital cerebellar ataxia and intellectual disability (summary by Gulsuner et al., 2011). For a discussion of genetic heterogeneity of CAMRQ, see CAMRQ1 (224050).

Autosomal recessive spinocerebellar ataxia-34 (SCAR34) is characterized by the onset of slowly progressive cerebellar ataxia in infancy or early childhood. Affected individuals show motor delay with delayed walking (around 5 to 6 years), unsteady wide-based gait, dysarthria, dysmetria, nystagmus, abnormal smooth pursuit, intention tremor, and dysdiadochokinesia. Some patients may also have hypotonia, spasticity, or other movement abnormalities. Almost all patients have impaired intellectual development with speech delay, although the severity is highly variable. Brain imaging shows cerebellar atrophy (summary by Kaiyrzhanov et al., 2024). For a discussion of genetic heterogeneity of CAMRQ, see CAMRQ1 (224050).

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

The vision problems associated with this condition are congenital, which means they are present from birth. They tend to remain stable (stationary) over time.\n\nAutosomal recessive congenital stationary night blindness is a disorder of the retina, which is the specialized tissue at the back of the eye that detects light and color. People with this condition typically have difficulty seeing and distinguishing objects in low light (night blindness). For example, they may not be able to identify road signs at night or see stars in the night sky. They also often have other vision problems, including loss of sharpness (reduced acuity), nearsightedness (myopia), involuntary movements of the eyes (nystagmus), and eyes that do not look in the same direction (strabismus).

A very rare, complex form of hereditary spastic paraplegia characterised by a late-onset, slowly progressive spastic paraplegia associated with mild ataxia and dysarthria, upper extremity involvement (i.e. loss of finger dexterity, dysmetria), and mild cognitive impairment, without the presence of nystagmus. A hypomyelinating leucodystrophy and thin corpus callosum is observed in all cases and psychomotor development is normal or near normal. Caused by mutations in the GJC2 gene (1q41-q42) encoding the gap junction gamma-2 protein.

A rare partial autosomal trisomy/tetrasomy characterized by global developmental delay, intellectual disability, autistic behavior, muscular hypotonia, macrocephaly and facial dysmorphism (frontal bossing, short palpebral fissures, low set, dysplastic ears, short or shallow philtrum, high arched or narrow palate, micrognathia). Other associated clinical features include sleep disturbances, seizures, aplasia/hypoplasia of the corpus callosum, skeletal abnormalities (large hands and feet, long fingers and toes, talipes).

A rare, syndromic, hereditary optic neuropathy disorder characterized by early-onset, severe, progressive visual impairment, optic disc pallor and central scotoma, variably associated with dyschromatopsia, auditory neuropathy (e.g. mild progressive sensorineural hearing loss), sensorimotor axonal neuropathy and, occasionally, moderate hypertrophic cardiomyopathy.

A rare syndrome with characteristics of pre-natal onset growth retardation (low birth weight and short stature), hypotonia, developmental delay and intellectual disability associated with microcephaly and craniofacial (low anterior hairline, hypotelorism, thick lips with carp-shaped mouth, high-arched palate, low-set ears), cardiac (conotruncal heart malformations such as tetralogy of Fallot) and skeletal (hypoplastic thumbs and first metacarpals) abnormalities.

AP-4-associated hereditary spastic paraplegia (HSP), also known as AP-4 deficiency syndrome, is a group of neurodegenerative disorders characterized by a progressive, complex spastic paraplegia with onset typically in infancy or early childhood. Early-onset hypotonia evolves into progressive lower-extremity spasticity. The majority of children become nonambulatory and usually wheelchair bound. Over time spasticity progresses to involve the upper extremities, resulting in a spastic tetraplegia. Associated complications include dysphagia, contractures, foot deformities, dysregulation of bladder and bowel function, and a pseudobulbar affect. About 50% of affected individuals have seizures. Postnatal microcephaly (usually in the -2SD to -3SD range) is common. All have developmental delay. Speech development is significantly impaired and many affected individuals remain nonverbal. Intellectual disability in older children is usually moderate to severe.

Congenital disorders of glycosylation, previously called carbohydrate-deficient glycoprotein syndromes (CDGSs), are caused by defects in mannose addition during N-linked oligosaccharide assembly. CDGs can be divided into 2 types, depending on whether they impair lipid-linked oligosaccharide (LLO) assembly and transfer (CDG I), or affect trimming of the protein-bound oligosaccharide or the addition of sugars to it (CDG II) (Orlean, 2000). CDG Ic is characterized by psychomotor retardation with delayed walking and speech, hypotonia, seizures, and sometimes protein-losing enteropathy. It is the second largest subtype of CDG (summary by Sun et al., 2005). For a discussion of the classification of CDGs, see CDG1A (212065). Freeze and Aebi (1999) reviewed CDG Ib (602579) and CDG Ic.

A rare genetic multiple congenital anomalies/dysmorphic syndrome characterized by intellectual disability, obesity, macrocephaly, behavioral abnormalities (such as aggressive tantrums and autistic-like behavior), and delayed speech development. Dysmorphic facial features include large, square forehead, prominent supraorbital ridges, broad nasal tip, large ears, prominent lower lip, and minor dental anomalies such as small upper lateral incisors and central incisor gap.

Cardiospondylocarpofacial syndrome (CSCF) is characterized by growth retardation, dysmorphic facial features, brachydactyly with carpal-tarsal fusion, extensive posterior cervical vertebral synostosis, cardiac septal defects with valve dysplasia, and deafness with inner ear malformations (summary by Le Goff et al., 2016).

An X-linked dominant condition caused by mutation(s) in the IQSEC2 gene, encoding IQ motif and SEC7 domain-containing protein 2. It is characterized by substantially impaired intellectual functioning and behavioral abnormalities.

Congenital hypertryptophanemia, which is accompanied by hyperserotonemia, does not appear to have significant clinical consequences (Ferreira et al., 2017).

Bardet-Biedl syndrome is an autosomal recessive and genetically heterogeneous ciliopathy characterized by retinitis pigmentosa, obesity, kidney dysfunction, polydactyly, behavioral dysfunction, and hypogonadism (summary by Beales et al., 1999). Eight proteins implicated in the disorder assemble to form the BBSome, a stable complex involved in signaling receptor trafficking to and from cilia (summary by Scheidecker et al., 2014). Genetic Heterogeneity of Bardet-Biedl Syndrome BBS2 (615981) is caused by mutation in a gene on 16q13 (606151); BBS3 (600151), by mutation in the ARL6 gene on 3q11 (608845); BBS4 (615982), by mutation in a gene on 15q22 (600374); BBS5 (615983), by mutation in a gene on 2q31 (603650); BBS6 (605231), by mutation in the MKKS gene on 20p12 (604896); BBS7 (615984), by mutation in a gene on 4q27 (607590); BBS8 (615985), by mutation in the TTC8 gene on 14q32 (608132); BBS9 (615986), by mutation in a gene on 7p14 (607968); BBS10 (615987), by mutation in a gene on 12q21 (610148); BBS11 (615988), by mutation in the TRIM32 gene on 9q33 (602290); BBS12 (615989), by mutation in a gene on 4q27 (610683); BBS13 (615990), by mutation in the MKS1 gene (609883) on 17q23; BBS14 (615991), by mutation in the CEP290 gene (610142) on 12q21, BBS15 (615992), by mutation in the WDPCP gene (613580) on 2p15; BBS16 (615993), by mutation in the SDCCAG8 gene (613524) on 1q43; BBS17 (615994), by mutation in the LZTFL1 gene (606568) on 3p21; BBS18 (615995), by mutation in the BBIP1 gene (613605) on 10q25; BBS19 (615996), by mutation in the IFT27 gene (615870) on 22q12; BBS20 (619471), by mutation in the IFT172 gene (607386) on 9p21; BBS21 (617406), by mutation in the CFAP418 gene (614477) on 8q22; and BBS22 (617119), by mutation in the IFT74 gene (608040) on 9p21. The CCDC28B gene (610162) modifies the expression of BBS phenotypes in patients who have mutations in other genes. Mutations in MKS1, MKS3 (TMEM67; 609884), and C2ORF86 also modify the expression of BBS phenotypes in patients who have mutations in other genes. Although BBS had originally been thought to be a recessive disorder, Katsanis et al. (2001) demonstrated that clinical manifestation of some forms of Bardet-Biedl syndrome requires recessive mutations in 1 of the 6 loci plus an additional mutation in a second locus. While Katsanis et al. (2001) called this 'triallelic inheritance,' Burghes et al. (2001) suggested the term 'recessive inheritance with a modifier of penetrance.' Mykytyn et al. (2002) found no evidence of involvement of the common BBS1 mutation in triallelic inheritance. However, Fan et al. (2004) found heterozygosity in a mutation of the BBS3 gene (608845.0002) as an apparent modifier of the expression of homozygosity of the met390-to-arg mutation in the BBS1 gene (209901.0001). Allelic disorders include nonsyndromic forms of retinitis pigmentosa: RP51 (613464), caused by TTC8 mutation, and RP55 (613575), caused by ARL6 mutation.

MDDGB3 is an autosomal recessive congenital muscular dystrophy with impaired intellectual development and mild brain abnormalities (Clement et al., 2008). It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (Mercuri et al., 2009). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

MDDGB2 is an autosomal recessive congenital muscular dystrophy associated with impaired intellectual development and mild structural brain abnormalities (Yanagisawa et al., 2007). It is part of a group of similar disorders, collectively known as 'dystroglycanopathies,' resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239) (Godfrey et al., 2007). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

Frontonasal dysplasia-2 (FND2) is an autosomal recessive disorder characterized by variable degrees of alopecia, skull defects, hypertelorism, depressed nasal bridge and ridge with notched alae nasi, and abnormal central nervous system findings (summary by Kariminejad et al., 2014).

The nephronophthisis (NPH) phenotype is characterized by reduced renal concentrating ability, chronic tubulointerstitial nephritis, cystic renal disease, and progression to end-stage renal disease (ESRD) before age 30 years. Three age-based clinical subtypes are recognized: infantile, juvenile, and adolescent/adult. Infantile NPH can present in utero with oligohydramnios sequence (limb contractures, pulmonary hypoplasia, and facial dysmorphisms) or postnatally with renal manifestations that progress to ESRD before age 3 years. Juvenile NPH, the most prevalent subtype, typically presents with polydipsia and polyuria, growth retardation, chronic iron-resistant anemia, or other findings related to chronic kidney disease (CKD). Hypertension is typically absent due to salt wasting. ESRD develops at a median age of 13 years. Ultrasound findings are increased echogenicity, reduced corticomedullary differentiation, and renal cysts (in 50% of affected individuals). Histologic findings include tubulointerstitial fibrosis, thickened and disrupted tubular basement membrane, sporadic corticomedullary cysts, and normal or reduced kidney size. Adolescent/adult NPH is clinically similar to juvenile NPH, but ESRD develops at a median age of 19 years. Within a subtype, inter- and intrafamilial variability in rate of progression to ESRD is considerable. Approximately 80%-90% of individuals with the NPH phenotype have no extrarenal features (i.e., they have isolated NPH); ~10%-20% have extrarenal manifestations that constitute a recognizable syndrome (e.g., Joubert syndrome, Bardet-Biedl syndrome, Jeune syndrome and related skeletal disorders, Meckel-Gruber syndrome, Senior-Løken syndrome, Leber congenital amaurosis, COACH syndrome, and oculomotor apraxia, Cogan type).

A rare mitochondrial disease due to a defect in mitochondrial protein synthesis with a variable phenotype that includes onset in infancy or early childhood of failure to thrive and psychomotor regression (after initial normal development), as well as ocular manifestations (such as ptosis, nystagmus, optic atrophy, ophthalmoplegia and reduced vision). Additional manifestations include bulbar paresis with facial weakness, hypotonia, difficulty chewing, dysphagia, mild dysarthria, ataxia, global muscle atrophy, and areflexia. It has a relatively slow disease progression with patients often living into the third decade of life.

Myopathy, lactic acidosis, and sideroblastic anemia-2 (MLASA2) is an autosomal recessive disorder of the mitochondrial respiratory chain. The disorder shows marked phenotypic variability: some patients have a severe multisystem disorder from infancy, including cardiomyopathy and respiratory insufficiency resulting in early death, whereas others present in the second or third decade of life with sideroblastic anemia and mild muscle weakness (summary by Riley et al., 2013). For a discussion of genetic heterogeneity of MLASA, see MLASA1 (600462).

Cranioectodermal dysplasia (CED) is a ciliopathy with skeletal involvement (narrow thorax, shortened proximal limbs, syndactyly, polydactyly, brachydactyly), ectodermal features (widely spaced hypoplastic teeth, hypodontia, sparse hair, skin laxity, abnormal nails), joint laxity, growth deficiency, and characteristic facial features (frontal bossing, low-set simple ears, high forehead, telecanthus, epicanthal folds, full cheeks, everted lower lip). Most affected children develop nephronophthisis that often leads to end-stage kidney disease in infancy or childhood, a major cause of morbidity and mortality. Hepatic fibrosis and retinal dystrophy are also observed. Dolichocephaly, often secondary to sagittal craniosynostosis, is a primary manifestation that distinguishes CED from most other ciliopathies. Brain malformations and developmental delay may also occur.

19p13.13 deletion syndrome is a condition that results from a chromosomal change in which a small piece of chromosome 19 is deleted in each cell. The deletion occurs on the short (p) arm of the chromosome at a position designated p13.13.\n\nFeatures commonly associated with this chromosomal change include an unusually large head size (macrocephaly), tall stature, and intellectual disability that is usually moderate in severity. Many affected individuals have significantly delayed development, including speech, and children may speak few or no words. Weak muscle tone (hypotonia) and problems with coordinating muscle movement (ataxia) contribute to delays in gross motor skills (such as sitting and walking) and fine motor skills (such as holding a pencil).\n\nOther signs and symptoms that can occur with 19p13.13 deletion syndrome include seizures, abnormalities of brain structure, and mild differences in facial features (such as a prominent forehead). Many affected individuals have problems with feeding and digestion, including constipation, diarrhea, vomiting, and abdominal pain. Eye problems that can impair vision are also common. These include eyes that do not point in the same direction (strabismus) and underdevelopment of the optic nerves, which carry visual information from the eyes to the brain.\n\nThe signs and symptoms of 19p13.13 deletion syndrome vary among affected individuals. In part, this variation occurs because the size of the deletion, and the number of genes it affects, varies from person to person.

A form of congenital disorders of N-linked glycosylation with characteristics of facial dysmorphism (microcephaly, high forehead, low posterior hairline, strabismus), hypotonia, failure to thrive, intractable seizures, developmental delay, persistent vomiting and gastric bleeding. Additional features that may be observed include fat pads anomalies, inverted nipples, and body temperature oscillation. The disease is caused by mutations in the gene ALG11 (13q14.3).

A rare genetic multiple congenital anomalies/dysmorphic syndrome with the association of severe intellectual disability, strabismus and anterior maxillary protrusion with vertical maxillary excess, open bite and prominent crowded teeth. Mild cochlear hearing loss has been reported in addition.

Rubinstein-Taybi syndrome (RSTS) is characterized by distinctive facial features, broad and often angulated thumbs and halluces, short stature, and moderate-to-severe intellectual disability. The characteristic craniofacial features are downslanted palpebral fissures, low-hanging columella, high palate, grimacing smile, and talon cusps. Prenatal growth is often normal, then height, weight, and head circumference percentiles rapidly drop in the first few months of life. Short stature is typical in adulthood. Obesity may develop in childhood or adolescence. Average IQ ranges between 35 and 50; however, developmental outcome varies considerably. Some individuals with EP300-RSTS have normal intellect. Additional features include ocular abnormalities, hearing loss, respiratory difficulties, congenital heart defects, renal abnormalities, cryptorchidism, feeding problems, recurrent infections, and severe constipation.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Seckel syndrome is an autosomal recessive disorder characterized by proportionate short stature, severe microcephaly, mental retardation, and a typical 'bird-head' facial appearance (summary by Kalay et al., 2011). For a general phenotypic description and a discussion of genetic heterogeneity of Seckel syndrome, see 210600.

An autosomal recessive muscular dystrophy caused by mutations in the POMGNT1 gene. It is associated with characteristic brain and eye malformations, profound mental retardation, and death usually in the first years of life.

The NSDHL-related disorders include: CHILD (congenital hemidysplasia with ichthyosiform nevus and limb defects) syndrome, an X-linked condition that is usually male lethal during gestation and thus predominantly affects females; and CK syndrome, an X-linked disorder that affects males. CHILD syndrome is characterized by unilateral distribution of ichthyosiform (yellow scaly) skin lesions and ipsilateral limb defects that range from shortening of the metacarpals and phalanges to absence of the entire limb. Intellect is usually normal. The ichthyosiform skin lesions are usually present at birth or in the first weeks of life; new lesions can develop in later life. Nail changes are also common. The heart, lung, and kidneys can also be involved. CK syndrome (named for the initials of the original proband) is characterized by mild to severe cognitive impairment and behavior problems (aggression, attention deficit hyperactivity disorder, and irritability). All affected males reported have developed seizures in infancy and have cerebral cortical malformations and microcephaly. All have distinctive facial features, a thin habitus, and relatively long, thin fingers and toes. Some have scoliosis and kyphosis. Strabismus is common. Optic atrophy is also reported.

Fanconi anemia (FA) is characterized by physical abnormalities, bone marrow failure, and increased risk for malignancy. Physical abnormalities, present in approximately 75% of affected individuals, include one or more of the following: short stature, abnormal skin pigmentation, skeletal malformations of the upper and/or lower limbs, microcephaly, and ophthalmic and genitourinary tract anomalies. Progressive bone marrow failure with pancytopenia typically presents in the first decade, often initially with thrombocytopenia or leukopenia. The incidence of acute myeloid leukemia is 13% by age 50 years. Solid tumors – particularly of the head and neck, skin, and genitourinary tract – are more common in individuals with FA.

Fanconi anemia (FA) is characterized by physical abnormalities, bone marrow failure, and increased risk for malignancy. Physical abnormalities, present in approximately 75% of affected individuals, include one or more of the following: short stature, abnormal skin pigmentation, skeletal malformations of the upper and/or lower limbs, microcephaly, and ophthalmic and genitourinary tract anomalies. Progressive bone marrow failure with pancytopenia typically presents in the first decade, often initially with thrombocytopenia or leukopenia. The incidence of acute myeloid leukemia is 13% by age 50 years. Solid tumors – particularly of the head and neck, skin, and genitourinary tract – are more common in individuals with FA.

A heterozygous deletion of chromosome 15q11.2 may increase the susceptibility to neuropsychiatric or neurodevelopmental problems, including delayed psychomotor development, speech delay, autism spectrum disorder, attention deficit-hyperactivity disorder, obsessive-compulsive disorder, and possibly seizures (summary by Doornbos et al., 2009 and Burnside et al., 2011). See also chromosome 15q11.2 duplication syndrome (608636).

Raymond-type X-linked syndromic intellectual developmental disorder (MRXSR) is characterized by mildly to severely impaired intellectual development with speech and language difficulties associated with variable additional features, including marfanoid habitus, epilepsy, facial dysmorphism, hypotonia, and behavioral problems (summary by Baker et al., 2015 and Schirwani et al., 2018).

Any non-syndromic X-linked intellectual disability in which the cause of the disease is a mutation in the DLG3 gene.

Kabuki syndrome (KS) is characterized by typical facial features (long palpebral fissures with eversion of the lateral third of the lower eyelid; arched and broad eyebrows; short columella with depressed nasal tip; large, prominent, or cupped ears), minor skeletal anomalies, persistence of fetal fingertip pads, mild-to-moderate intellectual disability, and postnatal growth deficiency. Other findings may include: congenital heart defects, genitourinary anomalies, cleft lip and/or palate, gastrointestinal anomalies including anal atresia, ptosis and strabismus, and widely spaced teeth and hypodontia. Functional differences can include: increased susceptibility to infections and autoimmune disorders, seizures, endocrinologic abnormalities (including isolated premature thelarche in females), feeding problems, and hearing loss.

Acrodysostosis-1 (ACRDYS1) is a form of skeletal dysplasia characterized by short stature, severe brachydactyly, facial dysostosis, and nasal hypoplasia. Affected individuals often have advanced bone age and obesity. Laboratory studies show resistance to multiple hormones, including parathyroid, thyrotropin, calcitonin, growth hormone-releasing hormone, and gonadotropin (summary by Linglart et al., 2011). However, not all patients show endocrine abnormalities (Lee et al., 2012). Genetic Heterogeneity of Acrodysostosis See also ACRDYS2 (614613), caused by mutation in the PDE4D gene (600129) on chromosome 5q12.

Syndromic optic atrophy, also known as DOA+ syndrome, is a neurologic disorder characterized most commonly by an insidious onset of visual loss and sensorineural hearing loss in childhood with variable presentation of other clinical manifestations including progressive external ophthalmoplegia (PEO), muscle cramps, hyperreflexia, and ataxia. There appears to be a wide range of intermediate phenotypes (Yu-Wai-Man et al., 2010).

Leber congenital amaurosis is also associated with other vision problems, including an increased sensitivity to light (photophobia), involuntary movements of the eyes (nystagmus), and extreme farsightedness (hyperopia). The pupils, which usually expand and contract in response to the amount of light entering the eye, do not react normally to light. Instead, they expand and contract more slowly than normal, or they may not respond to light at all.\n\nA specific behavior called Franceschetti's oculo-digital sign is characteristic of Leber congenital amaurosis. This sign consists of affected individuals poking, pressing, and rubbing their eyes with a knuckle or finger. Poking their eyes often results in the sensation of flashes of light called phosphenes. Researchers suspect that this behavior may contribute to deep-set eyes in affected children.\n\nIn very rare cases, delayed development and intellectual disability have been reported in people with the features of Leber congenital amaurosis. Because of the visual loss, affected children may become isolated. Providing children with opportunities to play, hear, touch, understand and other early educational interventions may prevent developmental delays in children with Leber congenital amaurosis.\n\nAt least 20 genetic types of Leber congenital amaurosis have been described. The types are distinguished by their genetic cause, patterns of vision loss, and related eye abnormalities.\n\nLeber congenital amaurosis, also known as LCA, is an eye disorder that is present from birth (congenital). This condition primarily affects the retina, which is the specialized tissue at the back of the eye that detects light and color. People with this disorder typically have severe visual impairment beginning at birth or shortly afterward. The visual impairment tends to be severe and may worsen over time.

Nephrotic syndrome type 5 (NPHS5) is an autosomal recessive disorder characterized by very early onset of progressive renal failure manifest as proteinuria with consecutive edema starting in utero or within the first 3 months of life. A subset of patients may develop mild ocular anomalies, such as myopia, nystagmus, and strabismus (summary by Hasselbacher et al., 2006). For a general phenotypic description and a discussion of genetic heterogeneity of nephrotic syndrome, see NPHS1 (256300).

Rafiq syndrome (RAFQS) is an autosomal recessive disorder characterized by variably impaired intellectual and motor development, a characteristic facial dysmorphism, truncal obesity, and hypotonia. The facial dysmorphism comprises prominent eyebrows with lateral thinning, downward-slanting palpebral fissures, bulbous tip of the nose, large ears, and a thin upper lip. Behavioral problems, including overeating, verbal and physical aggression, have been reported in some cases. Serum transferrin isoelectric focusing shows a type 2 pattern (summary by Balasubramanian et al., 2019).

Hyperphosphatasia with impaired intellectual development syndrome-3 (HPMRS3) is an autosomal recessive disorder characterized by severe intellectual disability, hypotonia with poor motor development, poor speech, and increased serum alkaline phosphatase (summary by Hansen et al., 2013). However, the severity of the disorder can also vary to include more mild intellectual impairment (Krawitz et al., 2013). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of HPMRS, see HPMRS1 (239300). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Adams-Oliver syndrome (AOS) is characterized by aplasia cutis congenita (ACC) of the scalp and terminal transverse limb defects (TTLD). ACC lesions usually occur in the midline of the parietal or occipital regions, but can also occur on the abdomen or limbs. At birth, an ACC lesion may already have the appearance of a healed scar. ACC lesions less than 5 cm often involve only the skin and almost always heal over a period of months; larger lesions are more likely to involve the skull and possibly the dura, and are at greater risk for complications, which can include infection, hemorrhage, or thrombosis, and can result in death. The limb defects range from mild (unilateral or bilateral short distal phalanges) to severe (complete absence of all toes or fingers, feet or hands, or more, often resembling an amputation). The lower extremities are almost always more severely affected than the upper extremities. Additional major features frequently include cardiovascular malformations/dysfunction (23%), brain anomalies, and less frequently renal, liver, and eye anomalies.

The chromosome 8q21.11 deletion syndrome is characterized by impaired intellectual development and common facial dysmorphic features (summary by Palomares et al., 2011).

SCN8A-related epilepsy with encephalopathy is characterized by developmental delay, seizure onset in the first 18 months of life (mean 4 months), and intractable epilepsy characterized by multiple seizure types (generalized tonic-clonic seizures, infantile spasms, and absence and focal seizures). Epilepsy syndromes can include Lennox-Gastaut syndrome, West syndrome, and epileptic encephalopathies (e.g., Dravet syndrome). Hypotonia and movement disorders including dystonia, ataxia, and choreoathetosis are common. Psychomotor development varies from normal prior to seizure onset (with subsequent slowing or regression after seizure onset) to abnormal from birth. Intellectual disability, present in all, ranges from mild to severe (in ~50% of affected individuals). Autistic features are noted in some. Sudden unexpected death in epilepsy (SUDEP) of unknown cause has been reported in approximately 10% of published cases. To date SCN8A-related epilepsy with encephalopathy has been reported in the literature in about 50 individuals.

Any Pitt-Hopkins-like syndrome in which the cause of the disease is a mutation in the NRXN1 gene.

Encephalopathy due to defective mitochondrial and peroxisomal fission-1 (EMPF1) is characterized by delayed psychomotor development and hypotonia that may lead to death in childhood. Many patients develop refractory seizures, consistent with an epileptic encephalopathy, and thereafter show neurologic decline. The age at onset, features, and severity are variable, and some patients may not have clinical evidence of mitochondrial or peroxisomal dysfunction (summary by Sheffer et al., 2016; Fahrner et al., 2016). Genetic Heterogeneity of Encephalopathy Due to Defective Mitochondrial And Peroxisomal Fission See also EMPF2 (617086), caused by mutation in the MFF gene (614785) on chromosome 2q36.

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Neurodevelopmental disorder with hypotonia, craniofacial abnormalities, and seizures (NEDHCS) is an autosomal recessive syndrome characterized primarily by hypotonia and poor feeding apparent in early infancy. Affected individuals have severe global developmental delay, early-onset intractable seizures, and recognizable craniofacial dysmorphism with skull abnormalities. The disorder is believed to be unique to the Amish population, where it exhibits a founder effect (summary by Ammous et al., 2021).

Congenital disorder of glycosylation type Ir (CDG1R) is an autosomal recessive disorder characterized by developmental delay, failure to thrive, feeding difficulties, hypotonia, and strabismus. Transferrin analysis demonstrates underglycosylation (summary by Pi et al., 2022). For a discussion of the classification of CDGs, see CDG1A (212065).

Infantile cerebellar-retinal degeneration (ICRD) is a severe autosomal recessive neurodegenerative disorder characterized by onset between ages 2 and 6 months of truncal hypotonia, athetosis, seizures, and ophthalmologic abnormalities, particularly optic atrophy and retinal degeneration. Affected individuals show profound psychomotor retardation, with only some achieving rolling, sitting, or recognition of family. Brain MRI shows progressive cerebral and cerebellar degeneration (summary by Spiegel et al., 2012). A subset of patients may have a milder phenotype with variable features, including ataxia, developmental delay, and behavioral abnormalities (Blackburn et al., 2020). Mutation in the ACO2 gene also causes isolated optic atrophy (OPA9; 616289).

Coffin-Siris syndrome (CSS) is classically characterized by aplasia or hypoplasia of the distal phalanx or nail of the fifth and additional digits, developmental or cognitive delay of varying degree, distinctive facial features, hypotonia, hirsutism/hypertrichosis, and sparse scalp hair. Congenital anomalies can include malformations of the cardiac, gastrointestinal, genitourinary, and/or central nervous systems. Other findings commonly include feeding difficulties, slow growth, ophthalmologic abnormalities, and hearing impairment.

Complete congenital stationary night blindness (cCSNB) is a clinically and genetically heterogeneous group of retinal disorders characterized by nonprogressive impairment of night vision, absence of the electroretinogram (ERG) b-wave, and variable degrees of involvement of other visual functions. Individuals with cCSNB and animal models of the disorder have an ERG waveform that lacks the b-wave because of failure to transmit the photoreceptor signal through the retinal depolarizing bipolar cells (summary by Peachey et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of congenital stationary night blindness, see CSNB1A (310500).

Fanconi anemia (FA) is characterized by physical abnormalities, bone marrow failure, and increased risk for malignancy. Physical abnormalities, present in approximately 75% of affected individuals, include one or more of the following: short stature, abnormal skin pigmentation, skeletal malformations of the upper and/or lower limbs, microcephaly, and ophthalmic and genitourinary tract anomalies. Progressive bone marrow failure with pancytopenia typically presents in the first decade, often initially with thrombocytopenia or leukopenia. The incidence of acute myeloid leukemia is 13% by age 50 years. Solid tumors – particularly of the head and neck, skin, and genitourinary tract – are more common in individuals with FA.

Fanconi anemia (FA) is characterized by physical abnormalities, bone marrow failure, and increased risk for malignancy. Physical abnormalities, present in approximately 75% of affected individuals, include one or more of the following: short stature, abnormal skin pigmentation, skeletal malformations of the upper and/or lower limbs, microcephaly, and ophthalmic and genitourinary tract anomalies. Progressive bone marrow failure with pancytopenia typically presents in the first decade, often initially with thrombocytopenia or leukopenia. The incidence of acute myeloid leukemia is 13% by age 50 years. Solid tumors – particularly of the head and neck, skin, and genitourinary tract – are more common in individuals with FA.

Craniofacial microsomia is a term used to describe a spectrum of abnormalities that primarily affect the development of the skull (cranium) and face before birth. Microsomia means abnormal smallness of body structures. Most people with craniofacial microsomia have differences in the size and shape of facial structures between the right and left sides of the face (facial asymmetry). In about two-thirds of cases, both sides of the face have abnormalities, which usually differ from one side to the other. Other individuals with craniofacial microsomia are affected on only one side of the face. The facial characteristics in craniofacial microsomia typically include underdevelopment of one side of the upper or lower jaw (maxillary or mandibular hypoplasia), which can cause dental problems and difficulties with feeding and speech. In cases of severe mandibular hypoplasia, breathing may also be affected.\n\nPeople with craniofacial microsomia usually have ear abnormalities affecting one or both ears, typically to different degrees. They may have growths of skin (skin tags) in front of the ear (preauricular tags), an underdeveloped or absent external ear (microtia or anotia), or a closed or absent ear canal; these abnormalities may lead to hearing loss. Eye problems are less common in craniofacial microsomia, but some affected individuals have an unusually small eyeball (microphthalmia) or other eye abnormalities that result in vision loss.\n\nAbnormalities in other parts of the body, such as malformed bones of the spine (vertebrae), abnormally shaped kidneys, and heart defects, may also occur in people with craniofacial microsomia.\n\nMany other terms have been used for craniofacial microsomia. These other names generally refer to forms of craniofacial microsomia with specific combinations of signs and symptoms, although sometimes they are used interchangeably. Hemifacial microsomia often refers to craniofacial microsomia with maxillary or mandibular hypoplasia. People with hemifacial microsomia and noncancerous (benign) growths in the eye called epibulbar dermoids may be said to have Goldenhar syndrome or oculoauricular dysplasia.

Fatty acid hydroxylase-associated neurodegeneration (FAHN) is characterized early in the disease course by central nervous system involvement including corticospinal tract involvement (spasticity), mixed movement disorder (ataxia/dystonia), and eye findings (optic atrophy, oculomotor abnormalities), and later in the disease course by progressive intellectual impairment and seizures. With disease progression, dystonia and spasticity compromise the ability to ambulate, leading to wheelchair dependence. Life expectancy is variable. FAHN is considered to be a subtype of neurodegeneration with brain iron accumulation (NBIA).

Spastic paraplegia-54 (SPG54) is a complicated form of spastic paraplegia, a neurodegenerative disorder affecting fibers of the corticospinal tract. Affected individuals have delayed psychomotor development, intellectual disability, and early-onset spasticity of the lower limbs. Brain MRI shows a thin corpus callosum and periventricular white matter lesions. Brain magnetic resonance spectroscopy shows an abnormal lipid peak (summary by Schuurs-Hoeijmakers et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see 270800.

A rare complex type of hereditary spastic paraplegia with characteristics of childhood onset of progressive spastic paraplegia associated with optic atrophy (with reduced visual acuity and central scotoma), ophthalmoplegia, reduced upper-extremity strength and dexterity, muscular atrophy in the lower extremities and sensorimotor neuropathy. Caused by mutations in the C12ORF65 gene (12q24.31) encoding probable peptide chain release factor C12ORF65, mitochondrial.

Among the Yakuts, an Asian population isolate that is located in the northeastern part of Siberia, Maksimova et al. (2010) ascertained an autosomal recessive short stature syndrome involving postnatal growth failure, small hands and feet, loss of visual acuity with abnormalities of color vision, abnormal nuclear shape in neutrophil granulocytes (Pelger-Huet anomaly; see 169400), and normal intelligence.

Pontine tegmental cap dysplasia (PTCD) refers to a neurologic condition characterized by a distinct pattern of hindbrain malformations apparent on brain imaging. The abnormalities affect the pons, medulla, and cerebellum. In neuroradiologic studies, the ventral side of the pons is flattened, whereas there is vaulting ('capping') of the dorsal pontine border into the fourth ventricle. Affected individuals show a variety of neurologic deficits, most commonly sensorineural deafness, impaired cranial nerve function, and variable psychomotor retardation (summary by Barth et al., 2007).

Ectopia lentis is defined as an abnormal stretching of the zonular fibers that leads to lens dislocation, resulting in acute or chronic visual impairment (Greene et al., 2010). Citing the revised Ghent criteria for Marfan syndrome, Loeys et al. (2010) proposed the designation 'ectopia lentis syndrome' (ELS) for patients with ectopia lentis and a mutation in the FBN1 gene who lack aortic involvement, to highlight the systemic nature of the condition and to emphasize the need for assessment of features outside the ocular system (see DIAGNOSIS). Genetic Heterogeneity of Isolated Ectopia Lentis An autosomal recessive form of isolated ectopia lentis (ECTOL2; 225100) is caused by mutation in the ADAMTSL4 gene (610113).

Coffin-Siris syndrome is a congenital malformation syndrome characterized by developmental delay, intellectual disability, coarse facial features, feeding difficulties, and hypoplastic or absent fifth fingernails and fifth distal phalanges. Other more variable features may also occur. Patients with ARID1A mutations have a wide spectrum of manifestations, from severe intellectual disability and serious internal complications that could result in early death to mild intellectual disability (summary by Kosho et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of Coffin-Siris syndrome, see CSS1 (135900). The chromosome 1p36.11 duplication syndrome, in which the ARID1A gene is duplicated, is characterized by impaired intellectual development, microcephaly, dysmorphic facial features, and hand and foot anomalies.

EXOSC3 pontocerebellar hypoplasia (EXOSC3-PCH) is characterized by abnormalities in the posterior fossa and degeneration of the anterior horn cells. At birth, skeletal muscle weakness manifests as hypotonia (sometimes with congenital joint contractures) and poor feeding. In persons with prolonged survival, spasticity, dystonia, and seizures become evident. Within the first year of life respiratory insufficiency and swallowing difficulties are common. Intellectual disability is severe. Life expectancy ranges from a few weeks to adolescence. To date, 82 individuals (from 58 families) with EXOSC3-PCH have been described.

Malan syndrome (MALNS) is clinically characterized by overgrowth, advanced bone age, macrocephaly, and dysmorphic facial features. Patients develop marfanoid habitus, with long and slender body, very low body mass, long narrow face, and arachnodactyly, with age. Impaired intellectual development and behavior anomalies are present (summary by Martinez et al., 2015).

Cerebellar dysfunction with variable cognitive and behavioral abnormalities (CECBA) is an autosomal dominant neurologic disorder with significant phenotypic heterogeneity, even within families. The disorder is most often diagnosed through genetic analysis with retrospective clinical phenotyping. Symptom onset is usually in early childhood, although later onset, even in adulthood, has been reported. Most affected individuals show global developmental delay from early childhood, particularly of motor and language skills. Many have mild intellectual disability; behavioral and psychiatric abnormalities such as autism and obsessive-compulsive disorder are also often observed. The movement disorder is prominent and may include cerebellar signs such as ataxia, tremor, dysmetria, poor coordination, and dysarthria. Other abnormal movements including spasticity, myoclonus, and dystonia have been reported, thus widening the phenotypic spectrum. Brain imaging is usually normal, but may show cerebellar atrophy or nonspecific white matter lesions. Variable dysmorphic facial features may also be present (summary by Thevenon et al., 2012; Jacobs et al., 2021; Wijnen et al., 2020).

Autosomal recessive spinocerebellar ataxia-13 (SCAR13) is an autosomal recessive neurologic disorder characterized by delayed psychomotor development beginning in infancy. Affected individuals show mildly to profoundly impaired intellectual development with poor or absent speech as well as gait and stance ataxia and hyperreflexia. Most individuals also have eye movement abnormalities. Brain MRI shows cerebellar atrophy and ventriculomegaly (Guergueltcheva et al., 2012).

PBD14B is an autosomal recessive peroxisome biogenesis disorder characterized clinically by mild intellectual disability, congenital cataracts, progressive hearing loss, and polyneuropathy (Ebberink et al., 2012), all of which had been observed in patients with mild peroxisomal biogenesis disorders (e.g., Kelley et al., 1986; Poll-The et al., 1987). Additionally, recurrent migraine-like episodes following mental stress or physical exertion, not a common feature in peroxisome disorders, was reported. Thoms and Gartner (2012) classified the disorder described by Ebberink et al. (2012) in their patient as a mild 'Zellweger syndrome (214100) spectrum' (ZSS) disorder. See PBD1B (601539) for a phenotypic description and discussion of genetic heterogeneity of less severe phenotypes on the Zellweger syndrome spectrum. See PBD9B (614879) for another atypical peroxisome biogenesis disorder.

PACS1 neurodevelopmental disorder (PACS1-NDD) is characterized by mild-to-severe neurodevelopmental delays. Language skills are more severely affected than motor skills. Hypotonia is reported in about a third of individuals and is noted to improve over time. Approximately 60% of individuals are ambulatory. Feeding difficulty is common, with 25% requiring gastrostomy tube to maintain appropriate caloric intake. Other common features include constipation, seizures, behavioral issues, congenital heart anomalies, short stature, and microcephaly. Common facial features include hypertelorism, downslanting palpebral fissures, bulbous nasal tip, low-set and simple ears, smooth philtrum, wide mouth with downturned corners, thin upper vermilion, and wide-spaced teeth. To date approximately 35 individuals with PACS1-NDD have been reported.

Autosomal recessive congenital stationary night blindness is a disorder of the retina, which is the specialized tissue at the back of the eye that detects light and color. People with this condition typically have difficulty seeing and distinguishing objects in low light (night blindness). For example, they may not be able to identify road signs at night or see stars in the night sky. They also often have other vision problems, including loss of sharpness (reduced acuity), nearsightedness (myopia), involuntary movements of the eyes (nystagmus), and eyes that do not look in the same direction (strabismus).\n\nThe vision problems associated with this condition are congenital, which means they are present from birth. They tend to remain stable (stationary) over time.

Alazami syndrome is an autosomal recessive disorder characterized by severe growth restriction present at birth, severely impaired intellectual development, and distinctive facial features. Some patients have been reported with skeletal and behavioral features (summary by Imbert-Bouteille et al., 2019).

GAND syndrome is a neurodevelopmental syndrome characterized by global developmental delay apparent from infancy, with motor delay and moderate to severely impaired intellectual development. Most patients have poor speech acquisition, especially expressive language development, and may manifest signs of speech apraxia. Affected individuals have hypotonia and feeding difficulties in infancy, as well as common dysmorphic features, such as macrocephaly, frontal bossing, hypertelorism, deep-set eyes, posteriorly rotated ears, and elongated wide nose with prominent nasal tip. More variable features may include seizures, cardiac abnormalities, and nonspecific findings on brain imaging (summary by Shieh et al., 2020).

CTNNB1 neurodevelopmental disorder (CTNNB1-NDD) is characterized in all individuals by mild-to-profound cognitive impairment and in up to 39% of reported individuals by exudative vitreoretinopathy, an ophthalmologic finding consistent with familial exudative vitreoretinopathy (FEVR). Other common findings include truncal hypotonia, peripheral spasticity, dystonia, behavior problems, microcephaly, and refractive errors and strabismus. Less common features include intrauterine growth restriction, feeding difficulties, and scoliosis.

Congenital hydrocephalus-2 (HYC2) is a congenital disorder with onset in utero. Affected individuals have hydrocephalus with variably dilated ventricles and variable neurologic sequelae. Some individuals have other brain abnormalities, including lissencephaly, thinning of the corpus callosum, and neuronal heterotopia. Most patients have delayed motor development and some have delayed intellectual development and/or seizures. Additional congenital features, including cardiac septal defects, iris coloboma, and nonspecific dysmorphic features, may be observed. Some patients die in utero, in infancy, or in early childhood, whereas others have long-term survival (summary by Shaheen et al., 2017). For a discussion of genetic heterogeneity of congenital hydrocephalus, see 233600.

Ichthyosis with confetti (IWC), also known as congenital reticular ichthyosiform erythroderma (CRIE), is a rare skin condition characterized by slowly enlarging islands of normal skin surrounded by erythematous ichthyotic patches in a reticulated pattern. The condition starts in infancy as a lamellar ichthyosis, with small islands of normal skin resembling confetti appearing in late childhood and at puberty. Histopathologic findings include band-like parakeratosis, psoriasiform acanthosis, and vacuolization of keratinocytes with binucleated cells in the upper epidermis, sometimes associated with amyloid deposition in the dermis. Ultrastructural abnormalities include perinuclear shells formed from a network of fine filaments in the upper epidermis (summary by Krunic et al., 2003).

A subtype of Charcot-Marie-Tooth type 4 with characteristics of childhood onset of slowly progressing, demyelinating sensorimotor neuropathy, focally folded myelin sheaths in nerve biopsy, reduced nerve conduction velocities and the typical Charcot-Marie-Tooth phenotype (i.e. distal muscle weakness and atrophy, and sensory loss). There is evidence this disease is caused by homozygous or compound heterozygous mutation in the SBF1 gene on chromosome 22q.

MED12-related disorders include the phenotypes of FG syndrome type 1 (FGS1), Lujan syndrome (LS), X-linked Ohdo syndrome (XLOS), Hardikar syndrome (HS), and nonspecific intellectual disability (NSID). FGS1 and LS share the clinical findings of cognitive impairment, hypotonia, and abnormalities of the corpus callosum. FGS1 is further characterized by absolute or relative macrocephaly, tall forehead, downslanted palpebral fissures, small and simple ears, constipation and/or anal anomalies, broad thumbs and halluces, and characteristic behavior. LS is further characterized by large head, tall thin body habitus, long thin face, prominent nasal bridge, high narrow palate, and short philtrum. Carrier females in families with FGS1 and LS are typically unaffected. XLOS is characterized by intellectual disability, blepharophimosis, and facial coarsening. HS has been described in females with cleft lip and/or cleft palate, biliary and liver anomalies, intestinal malrotation, pigmentary retinopathy, and coarctation of the aorta. Developmental and cognitive concerns have not been reported in females with HS. Pathogenic variants in MED12 have been reported in an increasing number of males and females with NSID, with affected individuals often having clinical features identified in other MED12-related disorders.

Axenfeld-Rieger syndrome is an autosomal dominant disorder of morphogenesis that results in abnormal development of the anterior segment of the eye, and results in blindness from glaucoma in approximately 50% of affected individuals (Fitch and Kaback, 1978). Systemic anomalies are associated, including dental hypoplasia, failure of involution of periumbilical skin, and maxillary hypoplasia (Alkemade, 1969). Genetic Heterogeneity of Axenfeld-Rieger Syndrome Linkage studies indicate that a second type of Axenfeld-Rieger syndrome maps to chromosome 13q14 (RIEG2; 601499). A third form of Axenfeld-Rieger syndrome (RIEG3; 602482) is caused by mutation in the FOXC1 gene (601090) on chromosome 6p25. See 109120 for a form of Axenfeld-Rieger syndrome associated with partially absent eye muscles, hydrocephalus, and skeletal abnormalities.

Craniosynostosis (CRS) is a primary abnormality of skull growth involving premature fusion of the cranial sutures such that the growth velocity of the skull often cannot match that of the developing brain. This produces skull deformity and, in some cases, raises intracranial pressure, which must be treated promptly to avoid permanent neurodevelopmental disability (summary by Fitzpatrick, 2013). Craniosynostosis-3 (CRS3) includes coronal, sagittal, and multisuture forms (Sharma et al., 2013). For discussion of genetic heterogeneity of craniosynostosis, see CRS1 (123100).

Macrocephaly refers to an abnormally enlarged head inclusive of the scalp, cranial bones, and intracranial contents. Macrocephaly may be due to megalencephaly (true enlargement of the brain parenchyma), and the 2 terms are often used interchangeably in the genetic literature (reviews by Olney, 2007 and Williams et al., 2008). Autosomal recessive macrocephaly/megalencephaly syndrome is characterized by an enlarged cranium apparent at birth or in early childhood. Affected individuals have intellectual disability and may have dysmorphic facial features resulting from the macrocephaly (summary by Alfaiz et al., 2014).

Deafness, dystonia, and cerebral hypomyelination is an X-linked recessive mental retardation syndrome characterized by almost no psychomotor development, dysmorphic facial features, sensorineural deafness, dystonia, pyramidal signs, and hypomyelination on brain imaging (summary by Cacciagli et al., 2013).

X-linked intellectual developmental disorder-98 (XLID98) is a neurodevelopmental disorder characterized by delayed psychomotor development, poor speech, behavioral abnormalities, poor overall growth, dysmorphic facial features, and often early-onset seizures. Some carrier females are unaffected, whereas other females with mutations are affected; males tend to be more severely affected than females. It is believed that the phenotypic variability and disease manifestations in female carriers results from skewed X-inactivation or cellular mosaicism (summary by de Lange et al., 2016).

Foveal hypoplasia is defined as the lack of foveal depression with continuity of all neurosensory retinal layers in the presumed foveal area. Foveal hypoplasia as an isolated entity is a rare phenomenon; it is usually described in association with other ocular disorders, such as aniridia (106210), microphthalmia (see 251600), albinism (see 203100), or achromatopsia (see 216900). All reported cases of foveal hypoplasia have been accompanied by decreased visual acuity and nystagmus (summary by Perez et al., 2014). For a discussion of genetic heterogeneity of foveal hypoplasia, see FVH1 (136520).

Complex cortical dysplasia with other brain malformations (CDCBM) is a disorder of aberrant neuronal migration and disturbed axonal guidance. Affected individuals have mild to severe mental retardation, strabismus, axial hypotonia, and spasticity. Brain imaging shows variable malformations of cortical development, including polymicrogyria, gyral disorganization, and fusion of the basal ganglia, as well as thin corpus callosum, hypoplastic brainstem, and dysplastic cerebellar vermis. Extraocular muscles are not involved (summary by Poirier et al., 2010). Genetic Heterogeneity of Complex Cortical Dysplasia with Other Brain Malformations See also CDCBM2 (615282), caused by mutation in the KIF5C gene (604593) on chromosome 2q23; CDCBM3 (615411), caused by mutation in the KIF2A gene (602591) on chromosome 5q12; CDCBM4 (615412), caused by mutation in the TUBG1 gene (191135) on chromosome 17q21; CDCBM5 (615763), caused by mutation in the TUBB2A gene (615101) on chromosome 6p25; CDCBM6 (615771), caused by mutation in the TUBB gene (191130) on chromosome 6p21; CDCBM7 (610031), caused by mutation in the TUBB2B gene (612850) on chromosome 6p25; CDCBM9 (618174), caused by mutation in the CTNNA2 gene (114025) on chromosome 2p12; CDCBM10 (618677), caused by mutation in the APC2 gene (612034) on chromosome 19p13; CDCBM11 (620156), caused by mutation in the KIF26A gene (613231) on chromosome 14q32; CDCBM12 (620316), caused by mutation in the CAMSAP1 gene (613774) on chromosome 9q34; CDCBM13 (614563), caused by mutation in the DYNC1H1 gene (600112) on chromosome 14q32; CDCBM14A (606854) and CDCBM14B (615752), caused by mutation in the ADGRG1 gene (604110) on chromosome 16q21; and CDCBM15 (618737), caused by mutation in the TUBGCP2 gene (617817) on chromosome 10q26. The designation CDCBM8 was previously used to represent a phenotype caused by mutation in the TUBA8 gene (see 605742.0001) on chromosome 22q11; the patients with this phenotype were subsequently found to have a homozygous mutation in the SNAP29 gene (604202.0002), also on chromosome 22q11, that may have been responsible for the disorder. The same mutation in SNAP29 causes a similar disorder, CEDNIK syndrome (609528). See also lissencephaly (e.g., LIS1, 607432), which shows overlapping features and may result from mutation in tubulin genes.

Cardiofaciocutaneous (CFC) syndrome is characterized by cardiac abnormalities (pulmonic stenosis and other valve dysplasias, septal defects, hypertrophic cardiomyopathy, rhythm disturbances), distinctive craniofacial appearance, and cutaneous abnormalities (including xerosis, hyperkeratosis, ichthyosis, keratosis pilaris, ulerythema ophryogenes, eczema, pigmented moles, hemangiomas, and palmoplantar hyperkeratosis). The hair is typically sparse, curly, fine or thick, woolly or brittle; eyelashes and eyebrows may be absent or sparse. Nails may be dystrophic or fast growing. Some form of neurologic and/or cognitive delay (ranging from mild to severe) is seen in all affected individuals. Neoplasia, mostly acute lymphoblastic leukemia, has been reported in some individuals.

Perrault syndrome is characterized by sensorineural hearing loss (SNHL) in males and females and ovarian dysfunction in females. SNHL is bilateral and ranges from profound with prelingual (congenital) onset to moderate with early-childhood onset. When onset is in early childhood, hearing loss can be progressive. Ovarian dysfunction ranges from gonadal dysgenesis (absent or streak gonads) manifesting as primary amenorrhea to primary ovarian insufficiency (POI) defined as cessation of menses before age 40 years. Fertility in affected males is reported as normal (although the number of reported males is limited). Neurologic features described in some individuals with Perrault syndrome include learning difficulties and developmental delay, cerebellar ataxia, and motor and sensory peripheral neuropathy.

MDDGB14 is an autosomal recessive congenital muscular dystrophy characterized by severe muscle weakness apparent in infancy and impaired intellectual development. Some patients may have additional features, such as microcephaly, cardiac dysfunction, seizures, or cerebellar hypoplasia. It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (summary by Carss et al., 2013). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

Multiple congenital anomalies-hypotonia-seizures syndrome is an autosomal recessive disorder characterized by neonatal hypotonia, lack of psychomotor development, seizures, dysmorphic features, and variable congenital anomalies involving the cardiac, urinary, and gastrointestinal systems. Most affected individuals die before 3 years of age (summary by Maydan et al., 2011). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of MCAHS, see MCAHS1 (614080). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Infantile hypotonia with psychomotor retardation and characteristic facies (IHPRF) is a severe autosomal recessive neurologic disorder with onset at birth or in early infancy. Affected individuals show very poor, if any, normal cognitive development. Some patients are never learn to sit or walk independently (summary by Al-Sayed et al., 2013). Genetic Heterogeneity of Infantile Hypotonia with Psychomotor Retardation and Characteristic Facies See also IHPRF2 (616801), caused by mutation in the UNC80 gene (612636) on chromosome 2q34; and IHPRF3 (616900), caused by mutation in the TBCK gene (616899) on chromosome 4q24.

The chromosome 3q13.31 deletion syndrome is characterized by marked developmental delay, characteristic facies with a short philtrum and protruding lips, and abnormal male genitalia (Molin et al., 2012). Patients with Primrose syndrome (PRIMS; 259050) exhibit features overlapping those of the chromosome 3q13.31 deletion syndrome but also have ossified ear cartilage, severe muscle wasting, and abnormalities of glucose metabolism resulting in insulin-resistant diabetes mellitus in adulthood. Primrose syndrome is caused by mutation in the ZBTB20 gene (606025) on chromosome 3q13.

A rare, genetic, neurodevelopmental disorder characterized by global developmental delay, borderline to severe intellectual disability, feeding difficulties, behavioral anomalies, vision anomalies and mild facial dysmorphism. Other associated features may include microcephaly, short stature, urogenital or palatal anomalies (e.g. cleft palate), minor cardiac defects, recurrent infections or hearing loss.

Developmental delay with autism spectrum disorder and gait instability is a rare, genetic, neurological disorder characterized by infant hypotonia and feeding difficulties, global development delay, mild to moderated intellectual disability, delayed independent ambulation, broad-based gait with arms upheld and flexed at the elbow with brisk walking or running, and limited language skills. Behavior patterns are highly variable and range from sociable and affectionate to autistic behavior.

Severe intellectual disability-short stature-behavioral abnormalities-facial dysmorphism syndrome is a rare, genetic, syndromic intellectual disability disorder characterized by severe intellectual disability with limited or absent speech and language, short stature, acquired microcephaly, kyphoscoliosis or scoliosis, and behavioral disturbances that include hyperactivity, stereotypy and aggressiveness. Facial dysmorphism, that typically includes sloping forehead, mild synophrys, deep-set eyes, strabismus, anteverted large ears, prominent nose and dental malposition, is also characteristic.

Any periventricular nodular heterotopia in which the cause of the disease is a mutation in the ERMARD gene.

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

PDE4D haploinsufficiency syndrome is a rare syndromic intellectual disability characterized by developmental delay, intellectual disability, low body mass index, long arms, fingers and toes, prominent nose and small chin.

Bosch-Boonstra-Schaaf optic atrophy syndrome (BBSOAS) is an autosomal dominant disorder characterized by delayed development, moderately impaired intellectual development, and optic atrophy. Most patients also have evidence of cerebral visual impairment. Dysmorphic facial features are variable and nonspecific (summary by Bosch et al., 2014).

Intellectual disability-facial dysmorphism syndrome due to SETD5 haploinsufficiency is a rare, syndromic intellectual disability characterized by intellectual disability of various severity, hypotonia, feeding difficulties, dysmorphic features, autism and behavioral issues. Growth retardation, congenital heart anomalies, gastrointestinal and genitourinary defects have been rarely associated.

Hermansky-Pudlak syndrome (HPS) is characterized by oculocutaneous albinism, a bleeding diathesis, and, in some individuals, pulmonary fibrosis, granulomatous colitis, or immunodeficiency. Ocular findings include reduced iris pigment with iris transillumination, reduced retinal pigment, foveal hypoplasia with significant reduction in visual acuity (usually in the range of 20/50 to 20/400), nystagmus, and increased crossing of the optic nerve fibers. Hair color ranges from white to brown; skin color ranges from white to olive and is usually a shade lighter than that of other family members. The bleeding diathesis can result in variable bruising, epistaxis, gingival bleeding, postpartum hemorrhage, colonic bleeding, and prolonged bleeding with menses or after tooth extraction, circumcision, and other surgeries. Pulmonary fibrosis, a restrictive lung disease, typically causes symptoms in the early thirties and can progress to death within a decade. Granulomatous colitis is severe in about 15% of affected individuals. Neutropenia and/or immune defects occur primarily in individuals with pathogenic variants in AP3B1 and AP3D1.

Hermansky-Pudlak syndrome (HPS) is characterized by oculocutaneous albinism, a bleeding diathesis, and, in some individuals, pulmonary fibrosis, granulomatous colitis, or immunodeficiency. Ocular findings include reduced iris pigment with iris transillumination, reduced retinal pigment, foveal hypoplasia with significant reduction in visual acuity (usually in the range of 20/50 to 20/400), nystagmus, and increased crossing of the optic nerve fibers. Hair color ranges from white to brown; skin color ranges from white to olive and is usually a shade lighter than that of other family members. The bleeding diathesis can result in variable bruising, epistaxis, gingival bleeding, postpartum hemorrhage, colonic bleeding, and prolonged bleeding with menses or after tooth extraction, circumcision, and other surgeries. Pulmonary fibrosis, a restrictive lung disease, typically causes symptoms in the early thirties and can progress to death within a decade. Granulomatous colitis is severe in about 15% of affected individuals. Neutropenia and/or immune defects occur primarily in individuals with pathogenic variants in AP3B1 and AP3D1.

Bestrophinopathies, the spectrum of ophthalmic disorders caused by pathogenic variants in BEST1, are typically characterized by retinal degeneration. The four recognized phenotypes are the three autosomal dominant disorders: Best vitelliform macular dystrophy (BVMD), BEST1 adult-onset vitelliform macular dystrophy (AVMD), and autosomal dominant vitreoretinochoroidopathy (ADVIRC); and autosomal recessive bestrophinopathy (ARB). Onset is usually in the first decade (except AVMD in which onset is age 30 to 50 years). Slow visual deterioration is the usual course. Choroidal neovascularization can occur in rare cases. ADVIRC is also associated with panophthalmic involvement including nanophthalmos, microcornea, hyperopia, and narrow anterior chamber angle with angle closure glaucoma.

A rare pure or complex form of hereditary spastic paraplegia with characteristics of onset in infancy of progressive lower limb spasticity, abnormal gait, increased deep tendon reflexes and extensor plantar responses that may be associated with intellectual disability. Additional signs such as contractures in the lower limbs, amyotrophy, clubfoot and optic atrophy, have also been reported. Caused by homozygous mutation in the NT5C2 gene on chromosome 10q24.

Any non-syndromic X-linked intellectual disability in which the cause of the disease is a mutation in the MID2 gene.

The nephronophthisis (NPH) phenotype is characterized by reduced renal concentrating ability, chronic tubulointerstitial nephritis, cystic renal disease, and progression to end-stage renal disease (ESRD) before age 30 years. Three age-based clinical subtypes are recognized: infantile, juvenile, and adolescent/adult. Infantile NPH can present in utero with oligohydramnios sequence (limb contractures, pulmonary hypoplasia, and facial dysmorphisms) or postnatally with renal manifestations that progress to ESRD before age 3 years. Juvenile NPH, the most prevalent subtype, typically presents with polydipsia and polyuria, growth retardation, chronic iron-resistant anemia, or other findings related to chronic kidney disease (CKD). Hypertension is typically absent due to salt wasting. ESRD develops at a median age of 13 years. Ultrasound findings are increased echogenicity, reduced corticomedullary differentiation, and renal cysts (in 50% of affected individuals). Histologic findings include tubulointerstitial fibrosis, thickened and disrupted tubular basement membrane, sporadic corticomedullary cysts, and normal or reduced kidney size. Adolescent/adult NPH is clinically similar to juvenile NPH, but ESRD develops at a median age of 19 years. Within a subtype, inter- and intrafamilial variability in rate of progression to ESRD is considerable. Approximately 80%-90% of individuals with the NPH phenotype have no extrarenal features (i.e., they have isolated NPH); ~10%-20% have extrarenal manifestations that constitute a recognizable syndrome (e.g., Joubert syndrome, Bardet-Biedl syndrome, Jeune syndrome and related skeletal disorders, Meckel-Gruber syndrome, Senior-Løken syndrome, Leber congenital amaurosis, COACH syndrome, and oculomotor apraxia, Cogan type).

Congenital disorder of glycosylation type Iy (CDG1Y) is an X-linked disorder characterized by developmental delay, speech delay, impaired intellectual development, muscular hypotonia, microcephaly, and distinctive facial features (summary by Johnsen et al., 2024).

Intellectual developmental disorder with language impairment and with or without autistic features (IDDLA) is a neurodevelopmental disorder characterized by global developmental delay with moderate to severe speech delay that particularly affects expressive speech. Most patients have articulation defects, but frank verbal dyspraxia is not observed. Common dysmorphic features include broad forehead, downslanting palpebral fissures, short nose with broad tip, relative macrocephaly, frontal hair upsweep, and prominent digit pads. Gross motor skills are also delayed. Some patients have autistic features and/or behavioral problems. All reported cases have occurred de novo (review by Le Fevre et al., 2013).

Pontocerebellar hypoplasia type 9 (PCH9) is an autosomal recessive neurodevelopmental and neurodegenerative disorder characterized by severely delayed psychomotor development, progressive microcephaly, spasticity, seizures, and brain abnormalities, including brain atrophy, thin corpus callosum, and delayed myelination (summary by Akizu et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

Autosomal recessive intellectual developmental disorder-43 (MRT43) is characterized by impaired intellectual development, poor language skills, short stature, and dysmorphic features. Some patients may have significant motor delays (summary by Gangfuss et al., 2022).

A rare genetic syndromic intellectual disability characterized by global developmental delay and borderline to severe intellectual disability, autism spectrum disorder with obsessive behavior, stereotypies, hyperactivity but frequently friendly and affable personality, feeding difficulties, short stature, muscular hypotonia, microcephaly, characteristic dysmorphic features (hypertelorism, high arched eyebrows, ptosis, deep and/or broad nasal bridge, broad/prominent nasal tip, short and/or upturned philtrum, narrow mouth, and micrognathia), and skeletal anomalies (kyphosis and/or scoliosis, arthrogryposis, slender habitus and extremities). Other clinical features may include hernias, congenital heart defects, cryptorchidism and seizures.

Pontocerebellar hypoplasia type 2E is an autosomal recessive neurodegenerative disorder characterized by profound mental retardation, progressive microcephaly, spasticity, and early-onset epilepsy (summary by Feinstein et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of pontocerebellar hypoplasia type 2, see PCH2A (277470).

ADNP-related disorder is characterized by hypotonia, severe speech and motor delay, mild-to-severe intellectual disability, and characteristic facial features (prominent forehead, high anterior hairline, wide and depressed nasal bridge, and short nose with full, upturned nasal tip) based on a cohort of 78 individuals. Features of autism spectrum disorder are common (stereotypic behavior, impaired social interaction). Other common findings include additional behavioral problems, sleep disturbance, brain abnormalities, seizures, feeding issues, gastrointestinal problems, visual dysfunction (hypermetropia, strabismus, cortical visual impairment), musculoskeletal anomalies, endocrine issues including short stature and hormonal deficiencies, cardiac and urinary tract anomalies, and hearing loss.

Colobomatous microphthalmia-rhizomelic dysplasia syndrome is a rare, genetic developmental defect during embryogenesis characterized by a range of developmental eye anomalies (including anophthalmia, microphthalmia, colobomas, microcornea, corectopia, cataract) and symmetric limb rhizomelia with short stature and contractures of large joints. Intellectual disability with autistic features, macrocephaly, dysmorphic features, urogenital anomalies (hypospadia, cryptorchidism), cutaneous syndactyly and precocious puberty may also be present.

Poretti-Boltshauser syndrome is an autosomal recessive disorder characterized by cerebellar dysplasia, cerebellar vermis hypoplasia, cerebellar cysts in most patients, high myopia, variable retinal dystrophy, and eye movement abnormalities. Affected individuals have delayed motor development and often have speech delay. Cognitive function can range from normal to intellectually disabled (summary by Aldinger et al., 2014).

CAGSSS, which comprises cataracts, growth hormone deficiency, sensory neuropathy, sensorineural hearing loss, and skeletal dysplasia, is an autosomal recessive multisystemic disorder with a highly variable phenotypic spectrum. Not all of these features are always present, and almost all the features may present at different times and/or become more apparent with age. The skeletal features are consistent with spondyloepimetaphyseal dysplasia (SEMD) (summary by Vona et al., 2018). One family had a distinctive presentation with infantile-onset intractable seizures and cortical abnormalities reminiscent of Leigh syndrome (see 256000). The correlation between genotype and phenotype remains unclear, but since the IARS2 gene is involved in mitochondrial function, heterogeneous manifestations can be expected (Takezawa et al., 2018).

SETBP1 haploinsufficiency disorder (SETBP1-HD) is characterized by hypotonia and mild motor developmental delay; intellectual abilities ranging from normal to severe disability; speech and language disorder; behavioral problems (most commonly attention/concentration deficits and hyperactivity, impulsivity), and refractive errors and strabismus. Typically children with SETBP1-HD whose intellect is in the normal or borderline range (IQ 80-90) were diagnosed following genetic testing for behavioral problems and/or severe speech and language disorders (respectively: the inability to produce sounds in words correctly, and deficits in the understanding and/or expression of words and sentences). To date, 47 individuals with SETBP1-HD have been reported.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the NDST1 gene.

Autosomal recessive spinocerebellar ataxia-17 (SCAR17) is a neurologic disorder characterized by onset of gait ataxia and cerebellar signs in early childhood. Patients also have variably impaired intellectual development (summary by Evers et al., 2016).

PURA-related neurodevelopmental disorders include PURA syndrome, caused by a heterozygous pathogenic sequence variant in PURA, and 5q31.3 deletion syndrome, caused by a genomic 5q31.3 deletion encompassing all or part of PURA. PURA-related neurodevelopmental disorders are characterized by moderate-to-severe neurodevelopmental delay with absence of speech in most and lack of independent ambulation in many. Early-onset problems can include hypotonia, hypothermia, hypersomnolence, feeding difficulties, excessive hiccups, recurrent central and obstructive apneas, epileptic seizures, abnormal nonepileptic movements (dystonia, dyskinesia, and dysconjugate eye movements), and abnormal vision. Congenital heart defects, urogenital malformations, skeletal abnormalities, and endocrine disorders occur, but are less common.

Cerebellofaciodental syndrome is an autosomal recessive neurodevelopmental disorder characterized by delayed development, intellectual disability, abnormal facial and dental findings, and cerebellar hypoplasia (summary by Borck et al., 2015).

Congenital nonprogressive cone-rod synaptic disorder (CRSD) is characterized by stable low vision, nystagmus, photophobia, a normal or near-normal fundus appearance, and no night blindness. Electroretinography shows an electronegative waveform response to scotopic bright flash, near-normal to subnormal rod function, and delayed and/or decreased to nonrecordable cone responses (Traboulsi, 2013; Khan, 2014).

Childhood-onset spasticity with hyperglycinemia is an autosomal recessive disorder characterized by onset of slowly progressive spasticity that results in impaired gait in the first decade of life. Imaging of the central nervous system shows leukodystrophy and/or lesions in the upper spinal cord. More variable features include visual defects and mild learning disabilities. Serum glycine is increased, but CSF glycine is only mildly increased or normal; serum lactate is normal. The disorder represents a form of 'variant' nonketotic hyperglycinemia and is distinct from classic nonketotic hyperglycinemia (NKH, or GCE; 605899), which is characterized by significantly increased CSF glycine. Several forms of 'variant' NKH, including SPAHGC, appear to result from defects of mitochondrial lipoate biosynthesis (summary by Baker et al., 2014).

Any Meier-Gorlin syndrome in which the cause of the disease is a mutation in the GMNN gene.

A rare genetic neurological disorder with characteristics of congenital microcephaly, severe intellectual disability, hypertonia at birth lessening with age, ataxia and specific dysmorphic facial features including hirsutism, low anterior hairline and bitemporal narrowing arched thick and medially sparse eyebrows, long eyelashes, lateral upper eyelids swelling and a skin fold partially covering the inferior eyelids, low-set posteriorly rotated protruding ears, anteverted nares and a full lower lip. Brain imaging shows partial to almost complete agenesis of the corpus callosum and variable degrees of cerebellar hypoplasia. Caused by homozygous mutation in the FRMD4A gene on chromosome 10p13.

Severe hypotonia-psychomotor developmental delay-strabismus-cardiac septal defect syndrome is a rare, genetic, non-dystrophic congenital myopathy disorder characterized by a neonatal-onset of severe generalized hypotonia associated with mild psychomotor delay, congenital strabismus with abducens nerve palsy, and atrial and/or ventricular septal defects. Cryptorchidism is commonly reported in male patients and muscle biopsy typically reveals increased variability in muscle fiber size.

Lamb-Shaffer syndrome is a neurodevelopmental disorder characterized by global developmental delay, intellectual disability, poor expressive speech, and mild dysmorphic facial features. Additional variable skeletal abnormalities may also be present (summary by Nesbitt et al., 2015).

Impaired intellectual development and distinctive facial features with or without cardiac defects (MRFACD) is an autosomal dominant, complex syndromic neurodevelopmental disorder characterized by delayed psychomotor development, poor speech acquisition, distinctive dysmorphic facial features, including frontal bossing, upslanting palpebral fissures, depressed nasal bridge with bulbous tip, and macrostomia. There is variable penetrance of cardiac malformations, ranging from no malformations to patent foramen ovale to septal defects and/or transposition of the great arteries (summary by Adegbola et al., 2015).

Spastic paraplegia and psychomotor retardation with or without seizures is an autosomal recessive complex neurodevelopmental disorder with onset in infancy. Affected children show hypotonia followed by severely impaired global development and significant motor disability. Most develop seizures in childhood and have speech delay. Other features, such as ocular abnormalities, foot deformities, hypoplasia of the corpus callosum, and decreased white matter, are more variable (summary by Hollstein et al., 2015).

Congenital symmetric circumferential skin creases is characterized by the folding of excess skin, which leads to ringed creases, primarily of the limbs. Affected individuals also exhibit intellectual disability, cleft palate, and dysmorphic features (summary by Isrie et al., 2015). For a discussion of genetic heterogeneity of congenital symmetric circumferential skin creases, see CSCSC1 (156610).

Palatal anomalies-widely spaced teeth-facial dysmorphism-developmental delay syndrome is a rare, genetic multiple congenital anomalies/dysmorphic syndrome characterized by global developmental delay, axial hypotonia, palate abnormalities (including cleft palate and/or high and narrow palate), dysmorphic facial features (including prominent forehead, hypertelorism, downslanting palpebral fissures, wide nasal bridge, thin lips and widely spaced teeth), and short stature. Additional manifestations may include digital anomalies (such as brachydactyly, clinodactyly, and hypoplastic toenails), a single palmar crease, lower limb hypertonia, joint hypermobility, as well as ocular and urogenital anomalies.

Congenital disorder of glycosylation type IIn (CDG2N) is an autosomal recessive severe multisystem developmental disorder characterized by delayed psychomotor development apparent from infancy, hypotonia, and variable additional features, such as short stature, seizures, visual impairment, and cerebellar atrophy. Serum transferrin analysis shows a CDG type II pattern (summary by Boycott et al., 2015 and Park et al., 2015). For a discussion of genetic heterogeneity of CDG type II, see CDG2A (212066).

Seizures, scoliosis, and macrocephaly/microcephaly syndrome (SSMS) is an autosomal recessive neurodevelopmental disorder characterized by global developmental delay apparent from early infancy, impaired intellectual development, behavioral problems, poor or absent speech, seizures, dysmorphic facial features with macro- or microcephaly, and skeletal abnormalities, including scoliosis and delayed bone age. Other features may include hypotonia, gastrointestinal problems, and exostoses (summary by Gentile et al., 2019).

Yuan-Harel-Lupski syndrome (YUHAL) is a complex neurodevelopmental disorder characterized by global developmental delay and early-onset peripheral neuropathy. The disorder comprises features of both demyelinating Charcot-Marie-Tooth disease type 1A (CMT1A; 118220), which results from duplication of the PMP22 gene on 17p12, and Potocki-Lupski syndrome (PTLS; 610883), which results from duplication of a slightly proximal region on 17p11.2 that includes the RAI1 gene. These 2 loci are about 2.5 Mb apart. The resultant YUHAL phenotype may be more severe in comparison to the individual contributions of each gene, with particularly early onset of peripheral neuropathy and features of both central and peripheral nervous system involvement (summary by Yuan et al., 2015).

Smith-Kingsmore syndrome (SKS) is a rare autosomal dominant syndromic intellectual disability syndrome characterized by macrocephaly, seizures, umbilical hernia, and facial dysmorphic features including frontal bossing, midface hypoplasia, small chin, hypertelorism with downslanting palpebral fissures, depressed nasal bridge, smooth philtrum, and thin upper lip (Smith et al., 2013; Baynam et al., 2015).

Senior-Loken syndrome-9 is an autosomal recessive disorder characterized by early-onset nephronophthisis and pigmentary retinopathy. Additional more variable features can include liver defects, skeletal anomalies, and obesity (summary by Bizet et al., 2015). For a phenotypic description and a discussion of genetic heterogeneity of Senior-Loken syndrome, see 266900.

Autosomal dominant cutis laxa-3 (ADCL3) is characterized by thin skin with visible veins and wrinkles, cataract or corneal clouding, clenched fingers, pre- and postnatal growth retardation, and moderate intellectual disability. In addition, patients exhibit a combination of muscular hypotonia with brisk muscle reflexes (Fischer-Zirnsak et al., 2015). For a general phenotypic description and discussion of genetic heterogeneity of autosomal dominant cutis laxa, see ARCL1 (123700).

Craniosynostosis is a primary abnormality of skull growth involving premature fusion of the cranial sutures such that the growth velocity of the skull often cannot match that of the developing brain. This produces skull deformity and, in some cases, raises intracranial pressure, which must be treated promptly to avoid permanent neurodevelopmental disability (summary by Fitzpatrick, 2013). Craniosynostosis-6 (CRS6) is a bicoronal form associated with bony defects in the sagittal, metopic, or lambdoid sutures (Twigg et al., 2015). For a discussion of genetic heterogeneity of craniosynostosis, see CRS1 (123100).

Neurodevelopmental disorder with hearing loss, seizures, and brain abnormalities (NEDHSB) is an autosomal recessive disorder characterized by severe neurologic impairment including impaired intellectual development, epilepsy, microcephaly, abnormal muscle tone, and sensorineural hearing loss. Most affected individuals are nonambulatory, cannot sit unassisted, and have no speech development. More variable features include feeding difficulties, poor growth, cortical visual impairment, spasticity, scoliosis, immunodeficiency, and thrombocytopenia (Tanaka et al., 2015).

An autosomal dominant condition caused by mutation(s) in the MYT1L gene, encoding myelin transcription factor 1-like protein. It is characterized by intellectual disability and mild dysmorphic facial features.

Basel-Vanagaite-Smirin-Yosef syndrome is an autosomal recessive multiple congenital anomaly disorder characterized by severely delayed psychomotor development resulting in mental retardation, as well as variable eye, brain, cardiac, and palatal abnormalities (summary by Basel-Vanagaite et al., 2015).

White-Sutton syndrome is a neurodevelopmental disorder characterized by a wide spectrum of cognitive dysfunction, developmental delays (particularly in speech and language acquisition), hypotonia, autism spectrum disorder, and other behavioral problems. Additional features commonly reported include seizures, refractive errors and strabismus, hearing loss, sleep disturbance (particularly sleep apnea), feeding and gastrointestinal problems, mild genital abnormalities in males, and urinary tract involvement in both males and females.

Arboleda-Tham syndrome (ARTHS) is an autosomal dominant disorder with the core features of impaired intellectual development, speech delay, microcephaly, cardiac anomalies, and gastrointestinal complications (summary by Kennedy et al., 2019).

CLIFAHDD is a congenital disorder characterized by congenital contractures of the limbs and face, resulting in characteristic facial features, hypotonia, and variable degrees of developmental delay. All reported cases have occurred de novo (summary by Chong et al., 2015).

Slow-channel congenital myasthenic syndrome (SCCMS) is a disorder of the postsynaptic neuromuscular junction (NMJ) characterized by early-onset progressive muscle weakness. The disorder results from kinetic abnormalities of the acetylcholine receptor channel, specifically from prolonged opening and activity of the channel, which causes prolonged synaptic currents resulting in a depolarization block. This is associated with calcium overload, which may contribute to subsequent degeneration of the endplate and postsynaptic membrane. Treatment with quinine, quinidine, or fluoxetine may be helpful; acetylcholinesterase inhibitors and amifampridine should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Female-restricted X-linked syndromic intellectual developmental disorder-99 (MRXS99F) is an X-linked dominant neurodevelopmental disorder characterized by delayed psychomotor development and mild to moderate intellectual disability. Affected females can have a wide range of additional congenital anomalies, including scoliosis, postaxial polydactyly, mild cardiac or urogenital anomalies, dysmorphic facial features, and mild structural brain abnormalities (summary by Reijnders et al., 2016).

X-linked syndromic intellectual developmental disorder-34 (MRXS34) is an X-linked recessive neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability with poor speech, dysmorphic facial features, and mild structural brain abnormalities, including thickening of the corpus callosum (summary by Mircsof et al., 2015).

X-linked syndromic intellectual developmental disorder-33 (MRXS33) is an X-linked recessive neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability, and characteristic facial features (summary by O'Rawe et al., 2015).

Microphthalmia with linear skin defects (MLS) syndrome is characterized by unilateral or bilateral microphthalmia and/or anophthalmia and linear skin defects, usually involving the face and neck, which are present at birth and heal with age, leaving minimal residual scarring. Other findings can include a wide variety of other ocular abnormalities (e.g., corneal anomalies, orbital cysts, cataracts), central nervous system involvement (e.g., structural anomalies, developmental delay, infantile seizures), cardiac concerns (e.g., hypertrophic or oncocytic cardiomyopathy, atrial or ventricular septal defects, arrhythmias), short stature, diaphragmatic hernia, nail dystrophy, hearing impairment, and genitourinary malformations. Inter- and intrafamilial variability is described.

Spastic paraplegia, intellectual disability, nystagmus, and obesity (SINO) is an autosomal dominant neurologic disorder characterized by rapid growth in infancy, global developmental delay, spastic paraplegia, variable ophthalmologic defects, and dysmorphic facial features (summary by Josifova et al., 2016).

Intellectual developmental disorder with dysmorphic facies and ptosis (IDDDFP) is an autosomal dominant neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability, delayed language, and dysmorphic facial features, most notably ptosis/blepharophimosis. Additional features may include poor growth, hypotonia, and seizures (summary by Mattioli et al., 2017). See also chromosome 3p deletion syndrome (613792).

EBF3 neurodevelopmental disorder (EBF3-NDD) is associated with developmental delay (DD) / intellectual disability (ID), speech delay, gait or truncal ataxia, hypotonia, behavioral problems, and facial dysmorphism. Variability between individuals with EBF3-NDD is significant. Although all affected children have DD noted in early infancy, intellect generally ranges from mild to severe ID, with two individuals functioning in the low normal range. Less common issues can include genitourinary abnormalities and gastrointestinal and/or musculoskeletal involvement. To date, 42 symptomatic individuals from 39 families have been reported.

Optic atrophy-11 (OPA11) is an autosomal recessive disorder characterized by delayed psychomotor development, intellectual disability, ataxia, optic atrophy, and leukoencephalopathy on brain imaging. Laboratory studies are consistent with mitochondrial dysfunction (summary by Hartmann et al., 2016). For a discussion of genetic heterogeneity of optic atrophy, see OPA1 (165500).

KMT2B-related dystonia (DYT-KMT2B) is a complex childhood-onset (mean age 7 years) movement disorder described to date in 39 individuals. It is characterized by a progressive disease course evolving commonly from lower-limb focal dystonia into generalized dystonia with prominent cervical, cranial, and laryngeal involvement. Communication difficulties, secondary to articulation difficulties and low speech volume, are common. Bulbar dysfunction leads to impaired swallowing. Intellectual disability (ID) / developmental delay (DD) are commonly reported. Additional findings can include eye movement abnormalities, skin changes, psychiatric comorbidities (attention-deficit/hyperactivity disorder, anxiety, depression, and obsessive-compulsive disorder), myoclonus, seizures, spasticity, and sensorineural hearing loss. Many affected individuals follow a similar disease course, though milder and atypical findings have been described.

PEBAT is an autosomal recessive neurodevelopmental disorder characterized by severely delayed psychomotor development apparent soon after birth or in infancy, profound intellectual disability, poor or absent speech, and seizures. Most patients are never able to walk due to hypotonia or spasticity. Brain imaging shows cerebral and cerebellar atrophy, thin corpus callosum, and secondary hypomyelination. The disorder shows progressive features, including microcephaly, consistent with a neurodegenerative process (summary by Miyake et al., 2016; Flex et al., 2016).

Early-onset progressive encephalopathy with brain edema and/or leukoencephalopathy-1 (PEBEL1) is an autosomal recessive severe neurometabolic disorder characterized by rapidly progressive neurologic deterioration that is usually associated with a febrile illness. Affected infants tend to show normal early development followed by acute psychomotor regression with ataxia, hypotonia, respiratory insufficiency, and seizures, resulting in coma and death in the first years of life. Brain imaging shows multiple abnormalities, including brain edema and signal abnormalities in the cortical and subcortical regions (summary by Kremer et al., 2016). Genetic Heterogeneity of PEBEL See also PEBEL2 (618321), caused by mutation in the NAXD gene (615910) on chromosome 13q34.

The core features of short stature-micrognathia syndrome (SSMG) are intrauterine growth restriction (IUGR), postnatal short stature that is often rhizomelic, and micrognathia. Other common features include preterm birth, microcephaly, developmental delay, and genitourinary malformations in males. Transient liver dysfunction and glycosylation abnormalities during illness, giant cell hepatitis, hepatoblastoma, and cataracts have also been observed. Inter- and intrafamilial phenotypic severity varies greatly, from a relatively mild disorder to intrauterine death or stillbirth (Ritter et al., 2022).

A rare genetic, multiple congenital anomalies syndrome characterized by short stature, hand brachydactyly with hypoplastic distal phalanges, global development delay, intellectual disability, and more variably seizures, obesity, and craniofacial dysmorphism that includes microcephaly, high forehead, flat face, hypertelorism, deep set eyes, flat nasal bridge, averted nostrils, long philtrum, thin lip vermilion, and short neck.

ZTTK syndrome (ZTTKS) is a severe multisystem developmental disorder characterized by delayed psychomotor development and intellectual disability. Affected individuals have characteristic dysmorphic facial features, hypotonia, poor feeding, poor overall growth, and eye or visual abnormalities. Most patients also have musculoskeletal abnormalities, and some have congenital defects of the heart and urogenital system. Brain imaging usually shows developmental abnormalities such as gyral changes, cortical and/or cerebellar atrophy, and thin corpus callosum (summary by Kim et al., 2016).

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Thauvin-Robinet-Faivre syndrome is an autosomal recessive disorder characterized by generalized overgrowth, mainly of height, and mildly delayed psychomotor development with mild or severe learning difficulties. More variable features may include congenital heart defects, kidney abnormalities, and skeletal defects. Patients may have an increased risk for Wilms tumor (summary by Akawi et al., 2016).

Developmental and epileptic encephalopathy-42 (DEE42) is a neurologic disorder characterized by the onset of various types of seizures in the first hours or days of life, although rare patients may have onset in the first weeks of life. The seizures tend to be refractory and associated with EEG abnormalities, including multifocal spikes and generalized spike-wave complexes. Affected infants show global developmental delay with severely impaired intellectual development. Other features may include axial hypotonia, peripheral hypertonia with hyperreflexia, tremor, ataxia, and abnormal eye movements (summary by the Epi4K Consortium, 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Any Meier-Gorlin syndrome in which the cause of the disease is a mutation in the CDC45 gene.

Neurodevelopmental disorder with microcephaly and gray sclerae (NEDMIGS) is a severe autosomal recessive disorder characterized by impaired global development with hypotonia often precluding independent ambulation, profoundly impaired intellectual development with poor or absent language, mild microcephaly, and abnormal visual fixation. Patients also have gray sclerae and may have coarse facial features. Most affected individuals have seizures; some may have brain imaging abnormalities (summary by Shaheen et al., 2016 and Froukh et al., 2020).

Pontocerebellar hypoplasia type 2F (PCH2F) is an autosomal recessive neurodevelopmental disorder characterized by progressive microcephaly and variable neurologic signs and symptoms, including cognitive and motor delay, poor or absent speech, seizures, and spasticity (summary by Breuss et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of pontocerebellar hypoplasia type 2, see PCH2A (277470).

Any spastic quadriplegia in which the cause of the disease is a mutation in the ADD3 gene.

RERE-related disorders are characterized by neurodevelopmental problems with or without structural anomalies of the eyes, heart, kidneys, and genitourinary tract and mild sensorineural hearing loss. Hypotonia and feeding problems are common among affected individuals. Developmental delay and intellectual disability range from mild to profound. Behavior problems may include attention-deficit/hyperactivity disorder, self-injurious behavior, and autism spectrum disorder. A variety of eye anomalies (coloboma, optic nerve anomalies, microphthalmia, and/or Peter's anomaly) and vision issues (myopia, anisometropia, astigmatism, exotropia, esotropia) have been reported. Congenital heart defects, most commonly septal defects, have also been described. Genitourinary abnormalities include vesicoureteral reflux, and cryptorchidism and hypospadias in males. Sensorineural hearing loss can be unilateral or bilateral.

GNB1 encephalopathy (GNB1-E) is characterized by moderate-to-severe developmental delay / intellectual disability, structural brain abnormalities, and often infantile hypotonia and seizures. Other less common findings include dystonia, reduced vision, behavior issues, growth delay, gastrointestinal (GI) problems, genitourinary (GU) abnormalities in males, and cutaneous mastocytosis.

Immunodeficiency-centromeric instability-facial anomalies syndrome-3 is an autosomal recessive disorder characterized by recurrent infections in childhood and variable dysmorphic facial features. Laboratory studies show hypomethylation of certain chromosomal regions. Additional features, including delayed development, are variable (summary by Thijssen et al., 2015). For a discussion of genetic heterogeneity of immunodeficiency-centromeric instability-facial anomalies syndrome, see ICF1 (242860).

Witteveen-Kolk syndrome (WITKOS) is an autosomal dominant disorder with characteristic distinctive facial features, microcephaly, short stature, and mildly impaired intellectual development with delayed cognitive and motor development and subtle anomalies on MRI-brain imaging (summary by Balasubramanian et al., 2021).

Midface hypoplasia, hearing impairment, elliptocytosis, and nephrocalcinosis is an X-linked recessive disorder with onset of features in early childhood. Anemia is sometimes present. Some patients may show mild early motor or speech delay, but cognition is normal (summary by Andreoletti et al., 2017).

Any non-syndromic X-linked intellectual disability in which the cause of the disease is a mutation in the FRMPD4 gene.

BCL11A-related intellectual disability (BCL11A-ID) is characterized by developmental delay / intellectual disability of variable degree, neonatal hypotonia, microcephaly, distinctive but variable facial characteristics, behavior problems, and asymptomatic persistence of fetal hemoglobin. Growth delay, seizures, and autism spectrum disorder have also been reported in some affected individuals.

Noonan syndrome-like disorder with loose anagen hair is characterized by facial features similar to those observed in Noonan syndrome (163950), including hypertelorism, ptosis, downslanting palpebral fissures, low-set posteriorly angulated ears, and overfolded pinnae. In addition, patients display short stature, frequently with growth hormone (GH; see 139250) deficiency; cognitive deficits; relative macrocephaly; small posterior fossa resulting in Chiari I malformation; hypernasal voice; cardiac defects, especially dysplasia of the mitral valve and septal defects; and ectodermal abnormalities, in which the most characteristic feature is the hair anomaly, including easily pluckable, sparse, thin, slow-growing hair (summary by Bertola et al., 2017). Reviews Komatsuzaki et al. (2010) reviewed the clinical manifestations of patients with Noonan syndrome, Costello syndrome (218040), and cardiofaciocutaneous syndrome (CFC; see 115150) compared to patients with mutations in the SHOC2 gene. They noted that although there is phenotypic overlap among the disorders, loose anagen/easily pluckable hair had not been reported in mutation-positive patients with Noonan, CFC, or Costello syndrome, and appeared to be a distinctive feature of SHOC2 mutation-positive patients. Genetic Heterogeneity of Noonan Syndrome-Like Disorder with Loose Anagen Hair NSLH2 (617506) is caused by mutation in the PPP1CB gene (600590) on chromosome 2p23.

Developmental and epileptic encephalopathy-51 (DEE51) is an autosomal recessive severe neurodevelopmental disorder characterized by onset of intractable seizures and hypotonia in the first days or weeks of life. Affected individuals have severely delayed psychomotor development and may show abnormal movements. Brain imaging shows nonspecific abnormalities, such as cerebral atrophy, cerebellar atrophy, and delayed myelination. Laboratory studies showed increased lactate, suggesting mitochondrial dysfunction (summary by Ait-El-Mkadem et al., 2017). For a discussion of genetic heterogeneity of DEE, see 308350.

CDK13-related disorder, reported in 43 individuals to date, is characterized in all individuals by developmental delay / intellectual disability (DD/ID); nearly all individuals older than age one year display impaired verbal language skills (either absent or restricted speech). Other common findings are recognizable facial features in some individuals, behavioral problems (autism spectrum disorder or autistic traits/stereotypies, attention-deficit/hyperactivity disorder), feeding difficulties in infancy, structural cardiac defects, and seizures.

Autosomal recessive cutis laxa type IIC (ARCL2C) is characterized by generalized skin wrinkling with sparse subcutaneous fat and dysmorphic progeroid facial features. Most patients also exhibit severe hypotonia as well as cardiovascular involvement (summary by Van Damme et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal recessive cutis laxa, see ARCL1A (219100).

MDCCAID is an autosomal recessive form of muscular dystrophy with onset of progressive muscle weakness in early childhood. Almost all patients also have early-onset cataracts, most have impaired intellectual development of varying severity, and some have seizures (summary by Wiessner et al., 2017 and Osborn et al., 2017).

Jansen-de Vries syndrome (JDVS) is an autosomal dominant neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability with speech delay, and behavioral abnormalities. Most patients have variable additional features, including feeding and gastrointestinal difficulties, high pain threshold and/or hypersensitivity to sound, and dysmorphic features, including mild facial abnormalities, strabismus, and small hands and feet (summary by Jansen et al., 2017).

AMC1 is an autosomal recessive severe neurologic disorder with onset in utero. Most affected individuals die in utero or are subject to pregnancy termination because of lack of fetal movements and prenatal evidence of contractures of virtually all joints. Those who survive have generalized contractures and hypotonia. The disorder is caused by a neurogenic defect and poor or absent myelin formation around peripheral nerves rather than by a muscular defect (summary by Xue et al., 2017). <Genetic Heterogeneity of Arthrogryposis Multiplex Congenita Also see AMC2 (208100), caused by mutation in the ERGIC1 gene (617946); AMC3 (618484), caused by mutation in the SYNE1 gene (608441); AMC4 (618776), caused by mutation in the SCYL2 gene (616365); AMC5 (618947), caused by mutation in the TOR1A gene (605204), AMC6 (619334), caused by mutation in the NEB gene (161650), and AMC7 (301127), caused by mutation in the THOC2 gene (300395).

Stankiewicz-Isidor syndrome (STISS) is a neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability, behavioral disorders, mild craniofacial anomalies, and variable congenital defects of the cardiac and/or urogenital systems (summary by Kury et al., 2017).

Neurodevelopmental disorder with midbrain and hindbrain malformations (NEDMHM) is an autosomal recessive disorder comprising impaired intellectual development, speech delay, mild microcephaly, and midbrain-hindbrain malformation (Ravindran et al., 2017).

The name HIST1H1E syndrome has been proposed as a mnemonic for the characteristic features of this emerging, recognizable phenotype: hypotonia; intellectual disability with behavioral issues; skeletal; testes (undescended) and thyroid; heart anomalies (most commonly atrial septal defect); and ectodermal issues (including sparse hair, thin nails, and abnormal dentition). In the 47 affected individuals reported to date, predominant findings were intellectual disability (ranging from mild to profound) and behavioral problems (combinations of anxiety/phobias, obsessive behaviors, attention-deficit/hyperactivity disorder, and autistic spectrum disorder/traits among others). Skeletal involvement can include scoliosis and decreased bone mineral density. Other findings in some include seizures, craniosynostosis, and hearing loss. Life expectancy does not appear to be reduced in HIST1H1E syndrome.

Gabriele-de Vries syndrome is characterized by mild-to-profound developmental delay / intellectual disability (DD/ID) in all affected individuals and a wide spectrum of functional and morphologic abnormalities. Intrauterine growth restriction or low birth weight and feeding difficulties are common. Congenital brain, eye, heart, kidney, genital, and/or skeletal system anomalies have also been reported. About half of affected individuals have neurologic manifestations, including hypotonia and gait abnormalities. Behavioral issues can include attention-deficit/hyperactivity disorder, anxiety, autism or autistic behavior, and schizoaffective disorder.

Autosomal recessive limb-girdle muscular dystrophy-18 (LGMDR18) is characterized by childhood-onset of proximal muscle weakness resulting in gait abnormalities and scapular winging. Serum creatine kinase is increased. A subset of patients may show a hyperkinetic movement disorder with chorea, ataxia, or dystonia and global developmental delay (summary by Bogershausen et al., 2013). Additional more variable features include alacrima, achalasia, cataracts, or hepatic steatosis (Liang et al., 2015; Koehler et al., 2017). For a discussion of genetic heterogeneity of autosomal recessive limb-girdle muscular dystrophy, see LGMDR1 (253600).

Birk-Landau-Perez syndrome (BILAPES) is an autosomal recessive syndromic developmental disorder characterized by global developmental delay apparent from infancy or early childhood. Some patients have developmental regression with loss of speech and motor skills, whereas other patients never achieve these milestones. More variable features may include hypotonia, poor overall growth, ataxia, dystonia, abnormal eye movements, and renal insufficiency (Perez et al., 2017; Kleyner et al., 2022).

ISCA1-related multiple mitochondrial dysfunctions syndrome (ISCA1-MMDS) is a severe neurodegenerative condition typically characterized by either no attainment of developmental milestones or very early loss of achieved milestones, seizures in early infancy, development of spasticity with exaggerated deep tendon reflexes, nystagmus, and risk for sensorineural hearing loss. Affected individuals may also demonstrate elevated blood lactate levels with an elevated lipid-lactate peak on brain MR spectroscopy. Further brain MRI findings may include extensive cerebral and cerebellar deep white matter hyperintensities, marked dilatation of the cerebral ventricles, and pachygyria. Prognosis is poor and most individuals succumb to an intercurrent illness in early childhood.

WDR26-related intellectual disability (ID) is characterized by developmental delay / intellectual disability, characteristic facial features, hypotonia, epilepsy, and infant feeding difficulties. To date 15 individuals, ages 24 months to 34 years, have been reported. Developmental delay is present in all individuals and ranges from mild to severe. All individuals have delayed speech. Although some begin to develop speech in the second year, others have remained nonverbal. Seizures, present in all affected individuals reported to date, can be febrile or non-febrile (tonic-clonic, absence, rolandic seizures); most seizures are self limited or respond well to standard treatment. Affected individuals are generally described as happy and socially engaging; several have stereotypies / autistic features (repetitive or rocking behavior, abnormal hand movements or posturing, and at times self-stimulation).

CAKUTHED is an autosomal dominant highly pleiotropic developmental disorder characterized mainly by variable congenital anomalies of the kidney and urinary tract, sometimes resulting in renal dysfunction or failure, dysmorphic facial features, and abnormalities of the outer ear, often with hearing loss. Most patients have global developmental delay (summary by Heidet et al., 2017 and Slavotinek et al., 2017).

Combined oxidative phosphorylation deficiency-32 is an autosomal recessive neurodegenerative disorder characterized by onset of delayed psychomotor development and developmental regression in infancy. Affected individuals have multiple variable symptoms, including poor or absent speech, inability to walk, and abnormal movements. Brain imaging shows T2-weighted abnormalities in the basal ganglia and brainstem consistent with Leigh syndrome (256000). Patient cells showed decreased activities of mitochondrial respiratory chain complexes, I, III, and IV, as well as impaired mitochondrial translation (summary by Lake et al., 2017). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Al Kaissi syndrome (ALKAS) is an autosomal recessive developmental disorder characterized by growth retardation, spine malformation, particularly of the cervical spine, dysmorphic facial features, and delayed psychomotor development with moderate to severe intellectual disability (summary by Windpassinger et al., 2017).

Pontocerebellar hypoplasia type 11 (PCH11) is an autosomal recessive neurodevelopmental disorder characterized by severely delayed psychomotor development with impaired intellectual development and poor speech, microcephaly, dysmorphic features, and pontocerebellar hypoplasia on brain imaging. Additional features are more variable (summary by Marin-Valencia et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

3-Methylglutaconic aciduria type IX (MGCA9) is an autosomal recessive disorder characterized by early-onset seizures, severely delayed psychomotor development and intellectual disability. Patients have hypotonia or spasticity, and laboratory investigations show increased serum lactate and 3-methylglutaconic aciduria, suggestive of a mitochondrial defect (summary by Shahrour et al., 2017). For a phenotypic description and a discussion of genetic heterogeneity of 3-methylglutaconic aciduria, see MGCA type I (250950).

NEMMLAS is an autosomal recessive multisystemic disorder characterized by delayed psychomotor development, intellectual disability, and abnormal motor function, including hypotonia, dystonia, ataxia, and spasticity. Patient tissues may show deficiencies in one or more of the mitochondrial oxidative phosphorylation (OXPHOS) enzymes, but this is not a constant finding (summary by Wortmann et al., 2017).

Galloway-Mowat syndrome is a renal-neurologic disease characterized by early-onset nephrotic syndrome associated with microcephaly, gyral abnormalities of the brain, and delayed psychomotor development. Most patients have dysmorphic facial features, often including hypertelorism, ear abnormalities, and micrognathia. Other features, such as arachnodactyly and visual impairment, are more variable. Most patients die in the first years of life (summary by Braun et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of GAMOS, see GAMOS1 (251300).

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

SHRF is an autosomal recessive disorder characterized by short stature, brachydactyly, dysmorphic facial features, hearing loss, and visual impairment. Onset of the hearing and visual abnormalities, including retinitis pigmentosa, varies from birth to the second decade. Patients have mild intellectual disability and mild cerebellar atrophy with myelination defects on brain imaging (summary by Di Donato et al., 2016).

NEDMIAL is a neurodevelopmental disorder characterized by delayed psychomotor development and hypotonia apparent from early infancy, resulting in feeding difficulties, ataxic gait or inability to walk, delayed or absent speech development, and impaired intellectual development, sometimes with behavioral abnormalities, such as hand-flapping. Additional common features may include sleep disorder, nonspecific dysmorphic facial features, and joint hyperlaxity (summary by Lessel et al., 2017 and Mannucci et al., 2021).

NDAGSCW is a neurodevelopmental disorder characterized by severely delayed psychomotor development apparent from infancy. Affected individuals have delayed and difficulty walking, intellectual disability, absent speech, and variable additional features, including hip dysplasia, tapering fingers, and seizures. Brain imaging shows decreased cortical white matter, often with decreased cerebellar white matter, thin corpus callosum, and thin brainstem (summary by Lamers et al., 2017).

Sphingosine phosphate lyase insufficiency syndrome (SPLIS) is characterized by varying combinations of steroid-resistant nephrotic syndrome (ranging from nonimmune fetal hydrops to adolescent onset), primary adrenal insufficiency (with or without mineralocorticoid deficiency), testicular insufficiency, hypothyroidism, ichthyosis, lymphopenia/immunodeficiency, and neurologic abnormalities that can include developmental delay, regression / progressive neurologic involvement, cranial nerve deficits, and peripheral motor and sensory neuropathy.

Seckel syndrome is a rare autosomal recessive disorder characterized by intrauterine growth retardation, dwarfism, microcephaly with mental retardation, and a characteristic 'bird-headed' facial appearance (Shanske et al., 1997). Genetic Heterogeneity of Seckel Syndrome Other forms of Seckel syndrome include SCKL2 (606744), caused by mutation in the RBBP8 gene (604124) on chromosome 18q11; SCKL4 (613676), caused by mutation in the CENPJ gene (609279) on chromosome 13q12; SCKL5 (613823), caused by mutation in the CEP152 gene (613529) on chromosome 15q21; SCKL6 (614728), caused by mutation in the CEP63 gene (614724) on chromosome 3q22; SCKL7 (614851), caused by mutation in the NIN gene (608684) on chromosome 14q22; SCKL8 (615807), caused by mutation in the DNA2 gene (601810) on chromosome 10q21; SCKL9 (616777), caused by mutation in the TRAIP gene (605958) on chromosome 3p21; SCKL10 (617253), caused by mutation in the NSMCE2 gene (617246) on chromosome 8q24; and SCKL11 (620767), caused by mutation in the CEP295 gene (617728) on chromosome 11q21. The report of a Seckel syndrome locus on chromosome 14q, designated SCKL3, by Kilinc et al. (2003) was found to be in error; see History section.

Noonan syndrome with multiple lentigines (NSML) is a condition in which the cardinal features consist of lentigines, hypertrophic cardiomyopathy, short stature, pectus deformity, and dysmorphic facial features including widely spaced eyes and ptosis. Multiple lentigines present as dispersed flat, black-brown macules, mostly on the face, neck, and upper part of the trunk with sparing of the mucosa. In general, lentigines do not appear until age four to five years but then increase to the thousands by puberty. Some individuals with NSML do not exhibit lentigines. Approximately 85% of affected individuals have heart defects, including hypertrophic cardiomyopathy (typically appearing during infancy and sometimes progressive) and pulmonary valve stenosis. Postnatal growth restriction resulting in short stature occurs in fewer than 50% of affected persons, although most affected individuals have a height that is less than the 25th centile for age. Sensorineural hearing deficits, present in approximately 20% of affected individuals, are poorly characterized. Intellectual disability, typically mild, is observed in approximately 30% of persons with NSML.