ESR1 and breast cancer: Molecular subtypes, defined by estrogen receptor (ER), progesterone receptor (PgR), and human epidermal growth factor receptor-2 (HER2) status, can classify BCs into three different subgroups [luminal-like, triple negative (TN), and HER2 positive] and are important for predicting prognosis and treatment benefits for breast cancer (8, 9).