BRCA1 and breast cancer: Over the study period, however, it is unlikely that more than a quarter of breast cancers nationally were tested for oestrogen receptor status (Dowsett M, pers comm), and we estimate that at most a third of ovarian cancers after breast cancer at the study ages might be in BRCA1 carriers (Ford, 2000), so the overall effect on the ovarian cancer relative risk in relation to tamoxifen in our data should have been small.