Based on these markers, breast cancer can be classified in luminal A (ER+ and/or PR+, HER-2− and Ki67low), luminal B (ER+ and/or PR+, HER-2− and Ki67high), luminal-HER-2 (ER+ and/or PR+, and HER-2+), HER2-enriched (ER−, PR−, HER-2+), basal-like (ER−, PR−, HER-2−, and EFGR+ or CK5/6+), and triple-negative phenotype (TNBC) (ER−, PR−, HER-2−) [13,14,15] (Figure 1). Here, ESR1 is linked to breast cancer.