p27 (Cyclin-dependent kinase inhibitor p27) is a regulator of cell cycle progression. p27 is involved in G1 arrest. P27 is a potent inhibitor of cyclin E- and cyclin A-CDK2 complexes. P27 forms a complex with cyclin type D-CDK4 complexes and is involved in the assembly, stability, and modulation of CCND1-CDK4 complex activation. P27 acts either as an inhibitor or an activator of cyclin type D-CDK4 complexes depending on its phosphorylation state and/or stoichometry. Defects in CDKN1B are the cause of multiple endocrine neoplasia type 4 (MEN4). Multiple endocrine neoplasia (MEN) syndromes are inherited cancer syndromes of the thyroid. MEN4 is a MEN-like syndrome with a phenotypic overlap of both MEN1 and MEN2.