Cyclin-Dependent Kinase Inhibitor 1B (CDKN1B) is a Kinesin-related motor protein necessary for mitotic spindle assembly and chromosome segregation. CDKN1B is expressed in all tissues with highest levels observed in skeletal muscle. CDKN1B is a potent inhibitor of Cyclin E- and Cyclin A-CDK2 complexes. CDKN1B forms a complex with Cyclin Type D-CDK4 complexes and is involved in the assembly, stability, and modulation of CCND1-CDK4 complex activation. In addition, CDKN1B acts as an inhibitor or an activator of Cyclin Type D-CDK4 complexes depending on its phosphorylation state and stoichometry.