CHEK2 and breast cancer: Breast cancer in families with syndromes such as Li-Fraumeni syndrome (resulting from p53 gene mutations) [2] and Cowden syndrome (the mutated PTEN gene) [3], however, are each estimated to account for less than 1% of hereditary breast cancer, and mutations in ATM (the gene mutated in ataxia telangiectasia) and CHEK2 are also predicted to account for only a small proportion of familial breast cancer [4,5].