NDP and Familial exudative vitreoretinopathy: In the present retrospective study involving different clinical sites, the probands and their family members were clinically examined and identified by standard diagnostic tests, albeit performed by different operators; this, together with the limited observed population, impedes critical analysis of the phenotypic differences associated with the variations in different genes of the Norrin/Frizzled signaling pathway in our group of FEVR patients.