CDK4 and breast cancer: In combination with an aromatase inhibitor or fulvestrant, CDK4/6 inhibitors (palbociclib, ribociclib, and abemaciclib) have been approved by the Food and Drug Administration (FDA) for the treatment of hormone receptor (HR)-positive, growth-factor-receptor-2-positive, locally advanced, or metastatic human epidermal (HER2)-negative breast cancer [24,25,26].