ESR1 and breast cancer: Standard breast cancer treatment involves non-specific chemotherapy, supplemented with targeted drugs based on the molecular subtype: (i) estrogen antagonists like tamoxifen, fulvestran, or aromatase inhibitors for ER-positive tumors; and (ii) anti-HER2 antibodies like trastuzumab and TKIs such as lapatinib for HER2-positive tumors.