Based on expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), breast cancer can be categorized into the molecular subtypes of HER2-positive (ER-negative, PR-negative, and HER2-positive), luminal A (ER-positive and/or PR-positive and HER2-negative), luminal B (ER-positive and/or PR-positive and HER2-positve), and triple-negative (ER-negative, PR-negative, and HER2-negative) (3). The gene discussed is ESR1; the disease is breast cancer.