Norrie disease is a severe X-linked recessive trait with the hallmark features of congenital blindness, deafness, and mental retardation. The disorder is caused by mutations in a gene encoding norrin, a small extracellular protein. To characterize the function of norrin and to study the pathophysiology of Norrie disease, a knockout mouse model was generated and examined. The results of these studies showed that abnormal retinal angiogenesis during development is one of the most prominent observations in mice lacking norrin. This causes severe retinal hypoxia, which leads to profound tissue damage. The disease phenotype was rescued by breeding knockout mice with transgenic animals with ectopic norrin expression in the lens. In addition, transgenic lenses induced proliferation of microvascular endothelial cells in coculture. These and other findings identified norrin as a key regulator of angiogenic processes in the retina. Most of the ocular symptoms in human patients may also be attributed to oxygen deficiency during retinal development, and the mouse lines significantly contributed to a better understanding of the primary events of this severe neurological disorder.