The CDK4/6 inhibitors (CDK4/6i) palbociclib, ribociclib, and abemaciclib have been approved by the FDA for the treatment of metastatic hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2−) breast cancer (Eggersmann et al, 2019). This evidence concerns the gene CDK4 and breast cancer.