ESR1 and breast cancer: An immunohistochemical staining proxy based on biomarkers categorizes breast cancer into five significant subtypes: (1) luminal A (ER-positive and/or PR positive, HER-2 negative); (2) luminal B (ER-positive and/or PR positive, HER-2 positive); (3) HER-2 overexpressing (ER-negative, PR negative, HER-2 positive); (4) basal-like (ER-negative, PR negative, HER-2 negative, cytokeratin 5/6 positive and/or epidermal growth factor receptor {EGFR} positive); and (5) normal breast-like tumors (unclassified) were negative for all five biomarkers.