From OMIM
Coffin-Siris syndrome is a multiple malformation syndrome characterized by mental retardation associated with coarse facial features, hypertrichosis, sparse scalp hair, and hypoplastic or absent fifth fingernails or toenails. Other more variable features may include poor overall growth, craniofacial abnormalities, spinal anomalies, and congenital heart defects (review by Vergano and Deardorff, 2014). Mutations in the ARID1B gene are the most common cause of Coffin-Siris syndrome (Wieczorek et al., 2013). Genetic Heterogeneity of Coffin-Siris Syndrome Forms of Coffin-Siris syndrome have been shown to be caused by mutations in genes encoding subunits of the SWI/SNF complex, also known as the BAF complex, which functions as a chromatin remodeling factor. These include CSS2 (614607), caused by mutation in the ARID1A gene (603024); CSS3 (614608), caused by mutation in the SMARCB1 gene (601607); CSS4 (614609), caused by mutation in the SMARCA4 gene (603254); CSS5 (616938), caused by mutation in the SMARCE1 gene (603111); CSS6 (617808), caused by mutation in the ARID2 gene (609539); CSS7 (618027), caused by mutation in the DPF2 gene (601671); CSS8 (618362), caused by mutation in the SMARCC2 gene (601734); CSS9 (615866), caused by mutation in the SOX11 gene (600898); CSS10 (618506), caused by mutation in the SOX4 gene (184430); CSS11 (618779), caused by mutation in the SMARCD1 gene (601735); and CSS12 (619325), caused by mutation in the BICRA gene (605690). A similar phenotype, Nicolaides-Baraitser syndrome (NCBRS; 601358), is also caused by mutation in a subunit of this complex, i.e., SMARCA2 (600014).  http://www.omim.org/entry/135900

From MedlinePlus Genetics
Coffin-Siris syndrome is a condition that affects several body systems. Although there are many variable signs and symptoms, hallmarks of this condition include developmental disability, abnormalities of the fifth (pinky) fingers or toes, and characteristic facial features.

Most affected individuals have mild to severe intellectual disability or delayed development of speech and motor skills such as sitting and walking. Another feature of Coffin-Siris syndrome is underdevelopment (hypoplasia) of the tips of the fingers or toes, or hypoplasia or absence of the nails. These abnormalities are most common on the fifth fingers or toes. 

In addition, most people with Coffin-Siris syndrome have facial features described as coarse. These features typically include a wide nose with a flat nasal bridge, a wide mouth with thick lips, and thick eyebrows and eyelashes. Affected individuals can have excess hair on other parts of the face and body (hirsutism), but scalp hair is often sparse. People with Coffin-Siris syndrome can have a range of facial features, and not all affected individuals have the typical features. In addition, people with this condition may have an abnormally small head (microcephaly).

Additionally, some infants and children with Coffin-Siris syndrome have frequent respiratory infections, difficulty feeding, and an inability to gain weight at the expected rate (failure to thrive). Other signs and symptoms that may occur in people with this condition include short stature, low muscle tone (hypotonia), and abnormally loose (lax) joints. Abnormalities of the eyes, brain, heart, and kidneys may also be present.  https://medlineplus.gov/genetics/condition/coffin-siris-syndrome