ESR1 and neoplasm: From a clinical perspective, it is classified into three major subtypes, that is, hormone receptor–positive tumor (HR+) (either estrogen receptor–positive (ER+) or progesterone receptor–positive (PR+) tumors), human epidermal growth factor receptor 2 overexpression (HER2+), and triple-negative breast cancer (TNBC) (Nagini, 2017).