ESR1 and breast cancer: Based on immunophenotypic characteristics, breast cancer may be divided into: (1) estrogen receptor (ER) + and/or progesterone receptor (PR)+, and HER2− tumors (luminal A); (2) ER+ and/or PR+, and HER2+ or HER2− tumors (luminal B); (3) ER−, PR−, and HER2− tumors (triple-negative); and (4) ER−, PR−, and HER2+ tumors (HER2-overexpressing) [4].