FZD4 and Norrie disease: Since first described in 2002, numerous variants in the FZD4 gene have been reported to cause autosomal dominant FEVR with variable intrafamilial and interfamilial expressivity. Within the FZD4 signaling pathway, rare variants in the NDP gene result in Norrie disease (ND). The NDP gene encodes Norrin, the ligand for the FZD4 receptor.