About 5% to 10% of all BC cases are due to inheritance of a susceptibility allele, consistent with transmission in an autosomal dominant fashion, and a substantial proportion of these are due to germline mutations of the two major highly penetrant cancer susceptibility genes, BRCA1 (OMIM, 113705; GenBank, U14680.1) [1,2] and BRCA2 (OMIM, 600185; GenBank, U43746.1) [3-5]. This evidence concerns the gene BRCA1 and breast cancer.