There are four main categories of breast cancer according to the presence/absence of molecular markers mentioned above: HER2+ (ER−, PR−, HER2+), luminal A (ER+ and/or PR+, HER2−, Ki-67<14%), luminal B (ER+ and/or PR+, HER2+ or HER2−, Ki-67 >14%) and triple-negative breast cancer (TNBC) (ER−, PR−, HER2−) (3). The gene discussed is ESR1; the disease is breast cancer.