Although less than 5% of unselected breast cancer patients have a BRCA1 or BRCA2 mutation, extensive mammographic density is common among women with breast cancer, and estimates of attributable risk show that densities in more than 50% of the breast may account for about a third of all breast cancers (Byrne et al, 1995; Boyd et al, 2007). This evidence concerns the gene BRCA1 and breast cancer.