Mutations in BRCA1/2 are most common, causing a significantly increased risk of breast cancer of about 50–70% (BRCA1) and 40–60% (BRCA2) [4,6,7,8] as well as of ovarian cancer of about 40–50% (BRCA1) and 15–25% (BRCA2) [7,8] by 70–80 years of age. Here, BRCA1 is linked to breast cancer.