This was a retrospective analysis of data from National Surgical and Adjuvant Breast and Bowel Project trials (NSABP) B-18, B-22, and B-25, which demonstrated that among patients who did not receive tamoxifen, 89% with an ER-positive PBC had an ER-positive CBC and 70% with an ER-negative primary breast cancer had an ER-negative contralateral breast cancer (odds ratio = 14.8, 95% P < 0.001). This evidence concerns the gene ESR1 and breast cancer.