ESR1 and breast cancer: Tamoxifen is a member of the selective estrogen receptor modulator (SERM) drug family and is approved by the FDA for the treatment of patients with estrogen receptor-positive (ER+) metastatic breast cancer, for adjuvant therapy of high-risk ER+/progesterone receptor-positive (PR+) breast cancer, and for chemoprevention in women at high risk of developing breast cancer [1, 2].