For example, outgrowth of the UB occurs when MM‐derived glial cell–derived neurotrophic factor (Gdnf) interacts with the Ret receptor tyrosine kinase (RTK) and the co‐receptor Grfα1, and knockout of any of these factors in mice leads to renal agenesis or aplasia (Schuchardt et al., 1994; Pichel et al., 1996; Sanchez et al., 1996; Treanor et al., 1996; Enomoto et al., 1998). Here, GDNF is linked to renal agenesis.