A recent twin study estimated the heritability of breast cancer to be 31% [5], but the combination of rare variants (e.g., in BRCA1, BRCA2) identified from linkage studies (summarized in [6]) and common single nucleotide polymorphisms (SNPs) at roughly 100 loci identified from genome-wide association studies (GWAS; summarized in [7]) explain only one-third of the excess familial risk of disease [8]. This evidence concerns the gene BRCA1 and breast cancer.