In addition to these phenotypes, p27 null mice also have adrenal gland hyperplasias and tumors (pheochromocytomas) and female sterility (Fero et al., 1996; Kiyokawa et al., 1996; Nakayama et al., 1996). Here, CDKN1B is linked to hereditary pheochromocytoma-paraganglioma.