Thus far, three CDK4/6 inhibitors (CDK4/6is) (palbociclib, ribociclib and abemaciclib) have been approved in combination with endocrine therapy for the treatment of hormone receptor-positive, human epidermal growth factor (EGF) receptor 2-negative (HR+, HER2−) breast cancer, and clinical trials are underway in a spectrum of other tissue types4. Here, CDK4 is linked to breast cancer.