BRCA1 and breast cancer: Specifically, among the set of 176 BRCA1 mutation carriers analysed by Mitchell et al, 36% developed breast cancer, compared with only 17% of our 94 BRCA1 carriers, hence it is possible that their group of carriers were biased towards women with denser breasts (and thus higher risk, if density modifies breast cancer risk in mutation carriers in the same way that it does in the general population).