ESR1 and breast cancer: Based on the expression of estrogen and progesterone hormone receptors (ER and PR, respectively) and human epidermal growth factor receptor 2 (HER2), breast cancers can be classified into four major clinical treatment groups: (i) tumors positive for ER, PR or both but negative for HER2, (ii) tumors positive for ER, PR or both and positive for HER2, (iii) tumors HER2-positive (positive for HER2 but negative for ER and PR) and (iv) triple-negative (TN) (negative for ER, PR and HER2) [3].