sentences:  Cdk4 also requires activation upon complex assembly with Cyclin D. This is accomplished by a Cdk activating kinase (CAK), which phosphorylates Cdk4 at threonine 172.The function of the Cyclin D/Cdk4 complex suggests an obvious link to cancer and tumorigenesis.A hallmark of cancer is the downregulation of such inhibitory proteins as p53 and p27.