Since Gdnf is the major signal and acts primarily through the Ret receptor for UB induction [6–10], the reduced Gdnf expression likely contributes to the reduction in UB branching in the Osr1+/-Wt1+/- mouse embryos and is likely part of the mechanism underlying renal hypoplasia in infants heterozygous for the OSR1rs12329305(T) allele. This evidence concerns the gene WT1 and renal hypoplasia.