ESR1 and breast cancer: Generally, based on the presence or absence of specific receptors on the cell surface, breast cancer can be classified into three main subtypes, estrogen receptor (ER) and/or progesterone receptor (PR) positive (Luminal A/B, based on Ki-67 expression), human epidermal growth factor receptor 2 (HER2) positive, and triple negative breast cancer (TNBC) (7).