CDK4 and breast carcinoma: CDK4/6, as well as their target protein cyclin D1, are involved in cell cycle regulation and have been implicated in the pathogenesis of breast cancer and potential development of endocrine resistance.5,6 Currently, there are 3 available CDK4/6 inhibitors (CDK4/6i) approved by the US Food and Drug Administration (FDA): palbociclib (Ibrance, Pfizer), ribociclib (Kisqali, Novartis), and abemaciclib (Verzenio, Eli Lilly).