The CDK4/6 inhibitors (CDK4/6i) Palbociclib, Ribociclib, and Abemaciclib have recently gained FDA-approval and shown success in the treatment of advanced metastatic hormone receptor-positive (HR+)/HER2-negative breast cancer [15], with several clinical trials now testing their effects in multiple tumour types. This evidence concerns the gene CDK4 and breast cancer.