CDK4 and breast carcinoma: There are three CDK4/6 inhibitors, palbociclib, ribociclib, and abemaciclib, that are approved for treating advanced HR-positive (HR+), HER2-negative (HER2−) breast cancer, either combined with an aromatase inhibitor (AI) as a first-line treatment option or combined with fulvestrant as a second-line treatment option.