CDK (Cyclin-dependent kinases) is a group of serine/threonine kinases. It is activated by binding to cyclin and participates in the regulation of cell cycle.
Most of the known cyclin-CDK complexes regulate the progression through the cell cycle. Animal cells contain at least nine CDKs, four of which, CDK1, 2, 3, and 4, are directly involved in cell cycle regulation. In mammalian cells, CDK1, with its partners cyclin A2 and B1, alone can drive the cell cycle. Another one, CDK7, is involved indirectly as the CDK-activating kinase. A cyclin-dependent kinase inhibitor (CKI) is a protein that interacts with a cyclin-CDK complex to block kinase activity, usually during G1 or in response to signals from the environment or from damaged DNA.