Patients with breast cancer can be categorized into three general groups: 1) triple negative breast cancer dominated by basal type breast cancer (i.e., estrogen receptor negative (ER−), progesterone receptor negative (PR−), and human epidermal growth factor receptor-2 negative (HER-2); 2) the HER-2 subtype (ER−/PR−/HER2+); and 3) ER/PR positive breast cancer (ER+/PR+), which can be further subtyped based on mRNA profiling into luminal A (ER+/PR+/HER2−) and luminal B (ER+/PR+/HER2+)5. The gene discussed is ESR1; the disease is breast cancer.