Cyclin-dependent kinases (CDKs) are serine/threonine kinases whose activity requires binding of specific cyclin subunits to play critical roles in the control of the cell cycle or transcription. CDKs generally contain a catalytic serine/threonine kinase subunit and a regulatory cyclin subunit, and their activity is dependent on their association with cyclins and endogenous inhibitors (Ink4, Cip/Kip inhibitors), as well as by positive and negative phosphorylation events. Cyclins are periodically synthesized and destroyed during the cell cycle, thereby regulating CDK activity in a timely manner. In mammals, a total of 11 classical CDKs (CDK1-11) and 2 newly proposed family members (CDK12 and CDK13), as well as other proteins with structure or function similar to that of CDKs have been identified to date.

>70% as determined by SDS-PAGE.