Since the initial study of Molatore and Pellegata [53], which linked a CDKN1B mutation to a specific form of MEN in rats, cases of human MEN1-like phenotype not bearing mutations in MEN1 and RET genes have emerged, gradually confirming the association of CDKN1B with a distinct human MEN syndrome named MEN type 4 (MEN4) [54]. Here, MEN1 is linked to multiple endocrine neoplasia.