BC tumors are classified based on morphology, gene, and immunophenotypic expression, and are divided into four molecular groups: luminal A (express estrogen receptors [ER+], progesterone receptors [PR+], Human epidermal growth factor receptor 2 [HER2−]), luminal B (ER+, PR+ or HER2+/−), HER2+ (ER−, PR−, HER2+), and triple-negative BC (TNBC or basal-like) (ER−, PR−, HER2-). The gene discussed is ESR1; the disease is breast cancer.