The risk of breast cancer is substantially elevated for women with an affected first-degree relative carrying a high-risk mutation in BRCA1 or BRCA2 [4-6] or a mutation in ATM [7] or TP53 [8-10], but high-risk mutations in these susceptibility genes are rare and explain only approximately 25% of the familial risk of breast cancer [11]. The gene discussed is ATM; the disease is breast carcinoma.