Breast cancers arising in BRCA1 mutation carriers are estrogen receptor negative in 80–90% of cases, conversely those arising in BRCA2 mutation carriers are usually estrogen receptor positive [3] In the NSABP-P1 chemoprevention trial [13] the estimated risk ratios for BRCA1 and BRCA2 mutation carriers who took tamoxifen versus placebo were 1.67 (95% confidence interval 0.32–10.7) and 0.38 (95% confidence interval 0.06–1.56) respectively. This evidence concerns the gene BRCA1 and breast cancer.