ESR1 and breast cancer: Most studies profiling gene expression signatures have classified BC into five major molecular subtypes: luminal A (estrogen receptor (ER)-positive, progesterone receptor (PR)-positive, human epidermal growth factor receptor 2 (HER2)-negative), luminal B (ER-positive, PR-positive, HER2-positive), HER2-enriched (ER-negative, PR- negative, HER2-positive), basal-like or triple-negative (ER-negative, PR-negative, HER2-negative), and normal breast-like tumors (unclassified) [6,7].