NDP and Alzheimer disease: Four genes have been identified as candidates for FEVR: mutations of the Norrie disease (ND) gene have been found in XL-FEVR (the ND gene encodes the Norrie disease protein [NDP], Norrin) [7-9]; mutations of the FZD4 gene encoding frizzled 4 have been observed in AD-FEVR [10,11]; and the LRP5 gene encoding low-density lipoprotein receptor-related protein 5 (LRP5) is reported to show mutations in both AD-FEVR [11] and AR-FEVR [12].