ESR1 and breast neoplasm: The majority of breast tumors express estrogen receptor α (ERα, ESR1) [1], which is targeted by selective ER modulators (SERMs, e.g., tamoxifen (TAM)) that competitively inhibit estradiol (E2) and other estrogens from binding and activating ERα’s transcriptional activity and by aromatase inhibitor (AIs, e.g., letrozole) that reduce endogenous estrogen levels [2].