Several biomarkers such as estrogen receptor (ER) [5], human epidermal growth factor receptor 2 (HER-2/neu) [6], carbohydrate antigen 15-3 (CA 15-3) [7], and carcinoembryonic antigen (CEA) [8] have been evaluated for prognosis, diagnosis, and treatment monitoring in breast cancer. The gene discussed is ESR1; the disease is breast cancer.