Mutations in the known high-risk genes, BRCA1, BRCA2, p53, ATM and PTEN, account for less than 25% of the familial risk for breast cancer, and in the Stockholm region the frequency of mutations in BRCA1 and BRCA2 is less than 10% in families with three or more cases with breast cancer (Arver et al, 2001; Thompson and Easton, 2004). Here, PTEN is linked to breast cancer.