A rare syndromic form of cerebellar dysgenesis with characteristics of moderate to severe intellectual deficit and cerebellar abnormalities. OPHN1 syndrome is very rare. To date, up to 12 families have been reported. Affected male patients present moderate to severe intellectual disability, hypotonia, severe developmental delay, early-onset complex partial or tonic-clonic seizures, strabismus, dysmetria and occasionally ataxia. Cryptorchidism and genital hypoplasia have been reported. Various mutations including deletions and splice site mutations in the OPHN1 gene (Xq12) have been reported in patients with this syndrome. Transmission appears to follow an X-linked semi-dominant pattern.