ESR1 and breast cancer: Immunohistochemistry and DNA microarray technology (cDNA) have identified five molecular subtypes of BC: 1) luminal A (ER-positive and/or PR-positive and HER2-negative); 2) luminal B (ER-positive and/or PR-positive and HER2-positive); 3) HER2 overexpression (ER and PR-negative and HER2-positive); 4) basal-like (ER/PR/HER2-negative, cytokeratin 5/6-positive and/or epidermal growth factor receptor-positive); 5) normal breast like (tumors that are not included in any of the previous categories) [1,2].