Breast cancer is a disease of complex origin with a strong genetic component. The incidence of breast cancer is very high in monozygotic twins of patients, and it is thought that a high proportion and perhaps the majority of breast cancers arise in a small number of genetically susceptible women. Recent studies in human genetics have discovered several intervals in the human genome containing inherited variants that are statistically associated with the propensity to develop breast cancer. The investigators plan to use this knowledge to design a genetic screening test to guide recommendations for breast cancer screening with mammography. If the small group of genetically susceptible women can be identified, more effective breast cancer screening strategies can be implemented. In contrast, a very large proportion of women who undergo yearly mammography are at exceptionally low risk from a genetic perspective. Using genomic guidance could eventually reconfigure the most efficacious strategy to screen women for early detection of breast cancer.