Breast cancer can be largely divided into multiple subtypes (e.g., estrogen receptor-positive [ER+] luminal A, ER+ luminal B, human epidermal growth factor receptor 2-positive [HER2+] and triple negative) based on gene expression profile analyses (Perou et al., 2000; Cancer Genome Atlas Network, 2012). The gene discussed is ESR1; the disease is breast carcinoma.