ESR1 and breast cancer: Briefly, breast cancers are mainly divided into: (1) luminal A (ER/PR+, HER2−); (2) luminal B (ER/PR+, HER2−); (3) (HER2)-enriched; and (4) basal-like triple negative breast cancer (TNBC) (ER−, PR−, HER2−), with the latter being the most aggressive form [73].