CDK4 also known as cyclin-dependent kinase 4 or cell division protein kinase 4 is a member of the Ser/Thr protein kinase family. The activity of CDK4 is restricted to the G1-S phase, which is controlled by the regulatory subunits D-type cyclins and CDK inhibitor p16. CDK4/Cyclin D complex phosphorylate and inhibit members of the retinoblastoma (RB) protein family including RB1 and regulate the cell-cycle during G1/S transition.