More recently, gene expression profiling led to identification of five main molecular subtypes of breast cancer: HER2 overexpression (ER−/PR−/HER2+), basal-like (ER−/PR−/HER2−/basal marker+), luminal A (ER+/PR+/HER2−/KI67-), luminal B (ER+/PR+/HER2−/KI67+ or ER+/PR+/HER2+/KI67+) and normal-like (ER+/PR+/HER2−/KI67-) [1]. The gene discussed is ESR1; the disease is breast cancer.