ESR1 and breast cancer: For convenience, BC can be divided into three main subtypes: (1) estrogen receptor (ER)-positive and progesterone receptor (PR)-positive and epidermal growth factor receptor 2 (HER2)-negative (70% of patients); (2) HER2-positive (15–20%); and (3) triple-negative, i.e., negative for all three markers (15%) [2].