CDK4 and breast cancer: Recently, the addition of a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) (i.e., palbociclib, ribociclib or abemaciclib) to HT became the standard of care for HR+/HER2− metastatic BC patients, and two of these agents (ribociclib and abemaciclib) also showed efficacy in the adjuvant setting [1,12,13,14].