ESR1 and breast cancer: There are at least five distinct molecular breast cancer subtypes: luminal A (ER+/PR+/HER2−/low Ki-67 low proliferation rate); luminal B (ER+/PR+/HER2−/+/high Ki-67 high proliferation rate); HER2-enriched (ER−/PR−/HER2+); and triple negative breast cancers/TNBCs (ER−/PR−/HER2− and positive for typical basal cell cytokines) [5].