We generated a line of transgenic mice using a yeast artificial chromosome containing the Ret mutation responsible for the multiple endocrine neoplasia type 2B syndrome (MEN 2B). The resulting animals did not develop any of the expected neoplasms associated with MEN 2B. Transgenic animals were then bred with animals lacking murine Ret (RetM) to further evaluate the function of human mutated Ret (RetH2B) in the murine context. Whereas mice lacking RetM exhibit intestinal aganglionosis and the absence of kidneys with other genitourinary anomalies, expression of the RetH2B transgene in RetM-deficient mice allowed significant renal development with a partial rescue of the enteric nervous system. These RetH2B-positive/Ret M-deficient mice exhibit normal Ret expression and survive longer than RetM-deficient mice, but still die at 3 to 5 days of age with evidence of enterocolitis. We conclude that the normal expression of a human Ret proto-oncogene with the MEN 2B mutation does not cause any features of MEN 2B in mice. Although the gene is normally expressed in the appropriate target tissues, there is incomplete phenotypic rescue in mice lacking murine Ret. These results suggest important interspecies differences between humans and mice in the function of the Ret oncogene.