ESR1 and neoplasm: The hazard ratios of MKI67 expression are consistently higher in both ESR1 expression-based (HR = 3.45, 95% CI: 2.08–5.73) and 23-gene signature based ER positive tumours) (HR = 3.99, 95% CI: 2.31–6.89) compared to IHC-based ER positive tumours (HR = 2.33, 95% CI: 1.48–3.68) (Supplement Table S1); Bootstrapping results suggested that the improved prognostic power of MKI67 was significant for both ESR1-expression defined ER positive tumours (P<0.001) and for the 23-gene expression signature defined ER positive tumours (P<0.001).