The oligophrenin 1 gene (OPHN1 OMIM 300127) encodes a Rho-GTPase-activating protein (Rho-GAP). The OPHN1 protein has been implicated in the maintenance of structural and functional integrity in neuronal synapses, and its loss of function has been associated with X-linked mental retardation. OPHN1 is located at Xq12 and contains 25 exons. Various types of mutations have been detected within the OPHN1 gene. Mutations in OPHN1 have been seen in male subjects with severe mental retardation, as well as in females with mild to moderate mental retardation; the majority of affected patients exhibit cerebellar hypoplasia on neuroimaging (Philip N et al. J Med Genet. 2003;40:441-446).