ESR1 and breast carcinoma: High expression of MKI67 is prognostic of poor outcome (HR, 1.19; 95% CI, 0.98 to 1.45) in the five cohorts, but in the E2197 (Combination Chemotherapy in Treating Women With Breast Cancer) ER-positive cohort it had the opposite effect, with an HR of 0.78 (95% CI, 0.62 to 1.00), indicating that low expression is prognostic of poor outcome (Data Supplement).