ESR1 and breast cancer: Luminal B breast cancers can be subdivided into HER2- and HER2+: the HER2- tumors are usually ER+ (lower expression than in luminal A tumors), have high proliferation rates, a high Ki67 index, a micropapillary and lobular pleimorphic histology, and exhibit an intermediate perognosis; luminal B, HER2+ breast cancers are usually ER+, PR+, have a high Ki67 index and an intermediate prognosis.