Mammalian Ret is activated by GDNF to instruct epithelial morphogenesis in the uteric duct of the kidney [31] and Ret-activating mutations have been implicated in a variety of human cancers including epithelial cancers (breast and lung) and multiple endocrine neoplasia (men2) [32]–[35]. Here, GDNF is linked to multiple endocrine neoplasia.