ESR1 and breast cancer: The selective CDK4/6 inhibitors, palbociclib (PD0332991), ribociclib (LEE011), and abemaciclib (LY2835219), were approved by the Food and Drug Administration (FDA) and the European Medicines Agency (EMA) for the therapy of ER+/HER2− advanced and metastatic BC (A/MBC), showing dose-dependent growth inhibition in ER+ BC.