ESR1 and metastatic neoplasm: Most breast tumors express estrogen receptor α (ER) and progesterone receptor (PR), and patients receive adjuvant endocrine therapies depending on menopausal status, namely tamoxifen (premenopausal) or aromatase inhibitors (postmenopausal), with CDK4/6, PI3K, and mTOR inhibitors for metastatic disease [3].