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. This evidence concerns the gene RB1 and neoplasm.