ESR1 and breast cancer: In recent years, targeted therapy has been used for breast cancer treatment, which includes HER2-targeted therapy (trastuzumab, pertuzumab), ER/PR-targeted therapy (tamoxifen, letrozole, exemestane), as well as tyrosine kinase, PARP, CDK4/6, and mTOR inhibitors (lapatinib, neratinib, ribociclib, tucatinib, and others) [9,10,11,12].