ESR1 and breast cancer: MKI67 high ER+/HER2- BC was associated with a higher pathological complete response (pCR) rate in four of the eight neoadjuvant chemotherapy cohorts.<h4>Conclusions</h4>High MKI67 expression identifies highly proliferative tumors that are associated with genomic instability and immune activity and is associated with higher pCR rates following neoadjuvant chemotherapy in ER+/HER2- BC.