ESR1 and breast cancer: Based on hormone receptor status (estrogen receptor [ER] or progesterone receptor [PR]) and human epidermal receptor 2 (HER2) status, BC can be divided into subtypes of hormone receptor-positive (ER+/PR+) BC, HER2+ BC, and triple-negative (TNBC, ER−/PR−/HER2−) BC [113].