CDKN1B and Down syndrome: Moreover, germline inactivating mutations in cyclin-dependent kinase inhibitor (CDKI) genes have been found in rare families with MEN1-like syndromes (with mutations in either CDKN1A, CDKN2B, or CDKN2C) or the MEN4 syndrome, a phenotypic MEN1 syndrome characterized by mutations in CDKN1B [50, 52, 53].