Following the approval by the Food and Drug Administration (FDA) of palbociclib in 2015 for the management of ER+/HER2− advanced recurrent breast cancer, CDK4/6 inhibitors—including palbociclib, ribociclib, and abemaciclib—have established themselves as standard therapeutic options for high-risk, early-stage postoperative ER+/HER2− breast cancer patients now in addition to those presenting with advanced disease [18]. Here, ESR1 is linked to breast cancer.