In addition, although activation of the Norrin-FZD4-β-catenin pathway requires the presence of either LRP5 or LRP6 in vitro [14], it is unclear what exact roles LRP5 and LRP6 play during retinal vascular development in vivo, and what cell types contribute to the incomplete vascularization in the retina of OPPG patients and Lrp5-/- mice. The gene discussed is FZD4; the disease is osteoporosis-pseudoglioma syndrome.