BRCA1 and breast cancer: However a retrospective study demonstrated a 50% reduction in risk of contralateral breast cancer for both BRCA1 (odds ratio = 0.50: 95% CI, 0.30–0.85) and BRCA2 (odds ratio = 0.42: 95% CI, 0.17–1.02) mutation carriers who took tamoxifen after their first breast cancer.