At present, the prognosis, classification, and treatment of breast cancer is dependent on tumor histological grading, lymph node stage, and tumor stage (TNM), as well as 3 major protein markers: estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor (EGF) receptor 2 (HER2) [2, 3]. The gene discussed is ESR1; the disease is breast cancer.