ESR1 and breast cancer: Based on the molecular subtypes (gene expression), BC can be classified into four main molecular subtypes: Luminal A (ER+ (ER: estrogen receptor] and/or PR+ (PR: progesterone receptor), HER2− (HER2: human epidermal growth factor receptor 2), and low Ki67 (<14%)); Luminal B (ER+ and/or PR+ and HER2+(luminal HER2 group); ER+ and/or PR+, HER2−, and high Ki67 (>14%)); HER2-enriched (ER−, PR−, and HER2+); and basal-like or triple-negative (TNBC) (ER−, PR−, HER2−, and CK5/6 and/or EGFR+) [5].