This classifies them into four clinical groups: luminal A (45%, low Ki67, ER+/PR+/HER2−), luminal B (25%, higher Ki67, ER+/PR+/−/HER2−), HER2+ (15%, HER2 gene amplification) and triple-negative (15%, lacking ER, PR and HER2) breast cancers. The gene discussed is ESR1; the disease is breast cancer.