According to gene expression profiling and immunophenotypic pattern, breast cancer can be further classified into different biologic subtypes: luminal A (ER+/PR+/HER2−, Ki67 < 14%), luminal B (ER+/PR+/HER2−, Ki67 ≥ 14%), luminal-HER2 (ER+/PR+/HER2+), HER2+ (ER−/PR−/HER2+), and triple negative (ER−/PR−/HER2−). This evidence concerns the gene ESR1 and breast cancer.