ESR1 and breast cancer: Breast cancer is classified according to the expression of prognostic markers, including estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) (Luminal A, Luminal B that are ER, PR, and HER2 positive), HER2 only positive, BL (expressing basal cytokeratin); triple negative (TN) (negative for all three receptors) [13].