Accordingly, there are four molecular subtypes of breast cancer: luminal A (ER+/PR+/Ki67 low < 14% or Ki67 intermediate 14–19%); luminal B (ER+/PR+/HER2−/+/Ki67 intermediate 14–19% or Ki67 high > 20%); basal-like or triple-negative (ER−/PR−/HER2−/Ki67); and HER2 overexpressing (ER−/PR−/HER2+/Ki67) [9,10,11,12]. The gene discussed is ESR1; the disease is breast cancer.