ESR1 and neoplasm: However, despite sharing some common traits, luminal A cancers (HER2–) are generally ER+, PR high and Ki67 low, resulting in low-grade, slow-proliferating neoplasms, whereas luminal B tumours (HER2– or HER2+) are typically ER+, PR variable and Ki67 variable, translating into more aggressive cancers with a higher proliferative rate.22