ESR1 and cancer: High Ki67 positivity is a biomarker used to estimate prognosis and distinguish ER-positive luminal A disease from ER-positive luminal B disease.25,26,27 Luminal subtype distinction is important, as patients with luminal A cancer have improved overall survival compared with those with luminal B cancer.26,28,29 To investigate potential interactions between Ki67 and parity status, we stained available primary tumor samples for Ki67 (eTable 3 and eFigure 3 in the Supplement).