BC is classified into 4 main molecular subtypes, with each having important prognostic and therapeutic associations: luminal A (estrogen (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative); luminal B (ER-positive, HER2-positive); HER2+ (ER-negative, HER2-positive); and triple negative breast cancer (TNBC) (ER-negative, HER2-negative, progesterone (PR)-negative) carcinoma (3). The gene discussed is ESR1; the disease is carcinoma.