Based on marker status, breast cancer can be grouped into at least four subtypes: luminal A (ER+, PR+/−, HER2−), luminal B (ER+, PR+/−, HER2+), HER2 (ER−, PR−, HER2+), and Basal (ER−, PR−, HER2−) (62, 63). Here, ESR1 is linked to breast cancer.