ESR1 and breast cancer: We used a Window of Opportunity Study (PRESTO–PRe-operative ESTradiOl Window of Opportunity Study in Post-Menopausal Women with Newly Diagnosed ER Positive Breast Cancer) to test the hypothesis that estrogen could induce a decrease in Ki67 or Risk of Recurrence Score (ROR) (BC360TM, NanoString Technologies Inc., Seattle, WA, USA) [27].