CDK4 and neoplasm: CDK4 and CDK6 are serine/threonine kinases whose activity is regulated by various mechanisms, positively by association with cyclin D (D1–D3) and negatively by binding to CDK inhibitors of the INK4 family10. Palbociclib (PD0332991) is a specific CDK4/6 inhibitor that arrests cell cycle progression in proliferating tumor cells, and tumors lacking RB1 have been shown to be refractory to its treatment11.