ESR1 and breast carcinoma: Clinically, breast cancer comprises three main subtypes: human epidermal growth factor receptor 2 (HER2) positive breast cancer, hormone receptor [estrogen receptor (ER) and/or progesterone receptor (PgR)]-positive breast cancer, and triple-negative breast cancer (TNBC), the latter of which lacks expression of ER, PgR, and HER2 [1, 2].