ESR1 and breast cancer: Indeed, therapeutic targeting of ER signaling for breast cancer management using selective ER modulators (SERMs; i.e., tamoxifen), aromatase inhibitors (i.e., anastrozole), or selective ER degraders (SERDS; i.e., fulvestrant) provides long-term benefit for the majority of patients with early-stage disease and extends the lives of many patients with advanced lesions.