Impaired mental functioning occurs as an isolated feature or as part of many syndromes listed in the X-linked catalog. Impaired intellectual development that is not associated with other distinguishing features is referred to as 'nonspecific.'
The Human Gene Mapping Nomenclature Committee (Mulley et al., 1992) proposed to designate each newly reported apparently unique X-linked mental retardation (MRX) family with gene symbols (e.g., MRX1, MRX2) if a minimal lod score of 2.0 was demonstrated between the MR locus and one or more X chromosome markers. [from
OMIM]