Breast cancer is the most common cancer in women, accounting for 25% of all new cases of cancer. Most cases are sporadic, while 5-10% are estimated to be due to an inherited predisposition. Autosomal dominant alterations in two genes, BRCA1 and BRCA2, are likely to account for most familial cases of early-onset breast and/or ovarian cancer and for 3-4% of all breast cancer. The lifetime risk of developing hereditary breast cancer and/or ovarian cancer can reach 80%. For a given mutation in the susceptibility gene, disease severity and age at onset show great variability within and between breast cancer families, suggesting the involvement of other genetic as well as non-genetic factors.