ESR1 and breast carcinoma: Based on immunohistochemical (IHC) analyses of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), breast cancer can be classified into four basic subtypes: hormone receptor (HR: ER or/and PR) + /HER2 − , HR − /HER2 + , HR + /HER2 + , and triple-negative (negative for ER, PR, and HER2), with frequencies of 68.9%, 7.5%, 10.2%, and 13.4%, respectively [14].