ESR1 and breast carcinoma: To date, four subtypes of invasive breast cancer have been identified: luminal subtype A, which exhibits high levels of estrogen receptor (ER) and progesterone receptor (PR), but low expression of human epidermal growth factor receptor 2 (HER2) and cell proliferation index; luminal subtype B, which exhibits ER/PR+, HER2, and high proliferation index; HER2+ breast cancer subtype; and triple-negative breast cancer (TNBC) subtype (2).