FZD4 and Familial exudative vitreoretinopathy: The cause of defective vascularization of the retina in FEVR is currently unclear; however, the fact that knockdown of FZD4 activity by either shRNA or expression of the patient-derived dominant negative allele in hRMECs causes a dramatic decrease in their capacity to tubularize (but did not alter growth) suggests an inability to vascularize or maintain vascular integrity as a major biological process defective in FEVR.