Based on the presence or absence of three molecular biomarkers, estrogen receptor-α (ER-α), progesterone receptor (PR), and human epidermal growth factor-2 (HER2), breast cancer is classified into five distinct molecular subtypes: (a) luminal A (positive for ER-α and/or PR while negative for HER-2); (b) luminal B (positive for ER-α and/or PR as well as HER2); (c) HER-2 overexpressing; (d) triple-negative; and (e) normal breast-like tumors [96,97]. Here, ESR1 is linked to breast cancer.