ESR1 and breast cancer: Clinically, breast cancer is divided into several different subgroups that are commonly classified based on the expression status of various receptors such as the estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), among which triple negative (ER-/PR-/HER2-) breast cancer (TNBC) reveals highly metastatic features and poor prognosis.