Depending on the expression of the estrogen receptor (ER), progesterone receptor (PR), Ki67 antigen, and Human epidermal growth factor receptor-2 (HER-2), BC can be divided into Luminal A&B (70% for HER-2 negative cases), HER-2 positive (around 15% including ER/PR + HER-2+ BC), and triple negative (TNBC) (10%–15%). The gene discussed is ESR1; the disease is breast cancer.