ESR1 and breast cancer: Selective estrogen receptor modulators (e.g., tamoxifen) and aromatase inhibitors (e.g., anatrozole, letrozole, and exemestane) are currently used for the treatment of these ER-positive breast cancers either alone or in combination with chemotherapy.2–6 Alternatively, ER-negative HER2-positive breast cancers can be treated with anti-HER2 drugs, (e.g., trastuzumab and lapatinib7–9).