ESR1 and breast cancer: Clinically, according to the expression differences of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), BC is divided into the following four types: Luminal A (ER+ and/or PR+, HER2−), Luminal B (ER+ and/or PR+, HER2−/+), HER2+ (ER−, PR−, HER2+) and triple-negative breast cancer (TNBC).