On the basis of these indicators, the various molecular subtypes of breast cancer are categorized as follows: luminal A (ER+ and/or PR+, HER2−, and Ki67 low), luminal B (ER+ and/or PR+, HER2−, and Ki67 high), luminal-HER2 (ER+ and/or PR+ and HER2+), HER2-enriched (ER−, PR−, HER2+), basal-like (ER−, PR−, HER2−, and EFGR+ or CK5/6+), and triple-negative (TN) phenotype (ER−, PR−, HER2−). This evidence concerns the gene ESR1 and breast cancer.