ESR1 and breast cancer: Breast cancer is heterogeneous and clinically classified into three main subtypes according to the status of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2): luminal subtype that expresses ER and/or PR, HER2-positive subtype that overexpresses HER2, and triple-negative breast cancer (TNBC) (2).