ESR1 and breast cancer: Up to the present day, multiple targeted drugs have been approved in the treatment of BC by FDA as illustrated in Table 1, including inhibitors of estrogen receptor (ER), aromatase, cyclin-dependent kinase (CDK) 4/6, mTOR (rapamycin), and poly(ADP-ribose) polymerase (PARP), and epithelial growth factor receptor (EGFR) and human epithelial growth factor receptor 2 (HER2)-targeted agents [3].