CDK6 also known as cell division protein kinase 6 is a member of the cyclin-dependent kinase family of Ser/Thr protein kinases. It is positively regulated primarily by its union to the D cyclins D1, D2 and D3. The activity of CDK6 first appears in mid-G1 phase, which is controlled by the Cyclin D and members of INK4 family of CDK inhibitors. CDK6, as well as CDK4, has been shown to phosphorylate, and thus regulate the activity of, tumor suppressor Retinoblastoma protein making CDK6 an important protein in cancer development.