Several tumor markers, such as carbohydrate antigen 15–3 (CA 15–3), thymidine kinase 1 (TK1), carcinoembryonic antigen (CEA), estrogen receptor (ER), and human epidermal growth factor receptor 2 (HER-2/neu), have been evaluated for their capacity to aid in the diagnosis, prognosis, and monitoring of breast cancer in humans [21-24]. Here, ESR1 is linked to breast cancer.