ESR1 and breast cancer: Gallen Conferences, breast cancer is classified into the following four subtypes: (1) triple-negative, which is defined as estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2)-negative; (2) HER2-enriched, which is defined as HER2-positive; (3) luminal A-like, which is defined as ER-positive and/or PR-positive, HER2-negative, low Ki67 labeling index; and (4) luminal B-like, which is defined as ER-positive and/or PR-positive, HER2-negative, and high Ki67 labeling index2.