ESR1 and breast cancer: Furthermore, based on phenotypic and genetic categorization BC has been separated into four different subgroups: 1) luminal A (ER+/PR+/HER2−, low proliferation factor Ki67+ (14%), low grade); 2) luminal B (ER+/PR+/HER2+, high proliferation factor Ki67+ (14%), high grade); 3) HER2-enriched (ER−/PR−/HER2+, any Ki67 level, strong proliferation); 4) basal-like (ER−/PR−/HER2−, high grade and proliferation, necrosis) [3, 4].