Breast cancer (BC) is primarily classified based on the differential expression of cell surface protein hormone receptors (HR)—estrogen or progesterone receptors (ER; PR) and human epidermal growth factor 2 (ERBB2; formerly HER2)—and is categorized into three major subtypes: ER+/PR+/ERBB2- (70% of patients), ERBB2+ or HER2+ (15–20%) and triple-negative breast cancer (TNBC or ER-/PR-/ERBB2- (15%)) [3]. The gene discussed is ESR1; the disease is breast cancer.