Cyclin-dependent kinase inhibitor p27 (p27Kip1) is regulator of cell cycle progression. p27Kip1 is involved in G1 arrest, and is a potent inhibitor of cyclin E- and cyclin A-CDK2 complexes. P27Kip1 forms a complex with cyclin type D-CDK4 complexes and is involved in the assembly, stability, and modulation of CCND1-CDK4 complex activation. 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.