ETs for breast cancer include selective ER modulators (SERMs; e.g., tamoxifen); gonadotropin-releasing hormone agonists (e.g., goserelin); aromatase inhibitors (AI; e.g., anastrozole, letrozole, exemestane); and the selective ER antagonist and degrader (SERD, selective estrogen receptor degrader) fulvestrant: The only SERD approved for first- and second-line metastatic ER+ (i.e., ER expressing) breast cancer (2–4). The gene discussed is ESR1; the disease is breast cancer.