ESR1 and breast cancer: Ki67 was measured at diagnosis and at surgery in the tumours of these patients and it was confirmed that Ki67 after 2-weeks’ treatment is more prognostic of 5-year recurrence risk than baseline Ki67 in ER + HER2- BC: recurrences were 60% lower for patients with baseline Ki67 ≥ 10% that fell below 10% after 2 weeks of AI (8.4% recurrence within 5 years) compared to patients whose Ki67 remained high (≥ 10%) after AI (21.7% recurrence within 5 years)12, suggesting intrinsic AI resistance is a major factor in ‘early’ recurrence (< 5 years).