NDP and Norrie disease: Norrin-receptor complex activates the canonical Wnt pathway.9,10,11,15,16 In case of a blockade in the Norrin binding pathway, the signaling is disturbed, resulting in a depression in target protein production.15 Although the NDP mutations are involved both in Norrie disease and FEVR, most mutations are related to Norrie disease, whereas a smaller percentage of these mutations cause X-linked FEVR.