CDK4 and cancer: PD0332991 (palbociclib), a specific inhibitor of CDK4 and CDK6 (CDK4/6) [7], shown to induce proliferation arrest and senescence in several different cancer types [8–11], was labeled a break through drug by the FDA in 2013 for its promising activity in estrogen receptor-positive breast cancer when combined with the aromatase inhibitor, letrozole.