FZD4 and retinopathy of prematurity: Given the small but significant effects of haploinsufficiency in Fzd4+/- mice under normoxic conditions, and to allow for comparison of the wild type and Fzd4+/- mice to human genetic data that suggested a small decrease in Norrin-FZD4 mediated vascular development could increase risk for severe ROP, we wanted to establish the minimum oxygen exposure (the critical tipping point) that elicited vaso-obliteration in mice.