Based on molecular differences primarily through gene expression and presence/absence of hormonal receptors on the surface of cell, four clinically relevant BC subtypes have been identified, including Luminal A (ER+, PR+, HER2−, Ki67 low expression), Luminal B (ER+, HER2−, PR−, or Ki67 high expression), TNBC (ER−, PR−, HER2−), and HER2 (HER2 overexpression) [4]. Here, ESR1 is linked to breast cancer.