Mutations in the norrin-encoding gene NDP cause Norrie disease (OMIM: #310600), an X-linked syndrome characterized by severe ocular malformations, including persistent fetal vasculature, retrolental fibrovascular masses, microphthalmia, and cataracts, as well as a variable occurrence of extraocular clinical manifestations, including sensorineural hearing loss and cognitive impairments [71,122]. The gene discussed is NDP; the disease is sensorineural hearing loss disorder.