A few years ago a novel multiple endocrine neoplasia syndrome, named multiple endocrine neoplasia type 4 (MEN4), was discovered thanks to studies conducted on a MEN syndrome in the rat (named MENX). The rat and the human syndromes are both caused by germline mutations in the Cdkn1b/CDKN1B gene, respectively. This gene encodes p27Kip1, a putative tumor suppressor which binds to and inhibits cyclin/cyclin-dependent kinase complexes, thereby preventing cell cycle progression. MEN4 patients carry heterozygous mutations at various residues of p27Kip1 and present with endocrine lesions mainly belonging to a MEN1-like spectrum: their most common phenotypic features are parathyroid and pituitary adenomas. Recently, germline mutations in p27kip1 were also identified in patients with a sporadic parathyroid disease presentation. In vitro functional analysis of several CDKN1B sequence changes identified in MEN4 patients detected impaired activity of the encoded p27Kip1 variant proteins (e.g. reduced expression, mislocalization or poor binding to interaction partners), thereby highlighting the characteristics of the protein which are critical for tumor suppression. Although the number of MEN4 patients is low, the discovery of this syndrome has demonstrated a novel role for CDKN1B as a tumor susceptibility gene for neuroendocrine tumors. Here, we review the clinical characteristics of the MEN4 syndrome and the molecular phenotype of the associated p27Kip1 mutations.