CDK4 and breast cancer: Three CDK4/6 inhibitors have been approved in metastatic HR+/HER2− BC: palbociclib, ribociclib, and abemaciclib as first-line treatments in association with AIs (PALOMA-2, MONALEESA-2, and MONARCH-3) or as second-line therapies associated with fulvestrant (PALOMA-3, MONALEESA-3, and MONARCH-2).