CDK

Cyclin-dependent kinases supply the cell-cycle engine of the CMGC group. Twenty human genes, CDK1 through CDK20, split into two functional tiers: CDK1, CDK2, CDK4 and CDK6 partner with A-, B-, D- and E-type cyclins to drive mitotic and G1/S transitions and to phosphorylate retinoblastoma protein, whereas CDK7, CDK8, CDK9, CDK12 and CDK13 phosphorylate the RNA polymerase II C-terminal domain to control transcription and mRNA processing. CDK5 is neuronal and activated by p35 or p39 instead of cyclins, while the PCTAIRE kinases CDK16-CDK18 and CDK20 remain poorly defined. Activation requires cyclin binding plus CAK-mediated T-loop phosphorylation. CDK4/6 inhibitors such as palbociclib validate the family therapeutically.