In women with a BRCA1 or BRCA2 mutation, prophylactic bilateral total mastectomy reduces the incidence of breast cancer at three years of follow-up.

).Mutations in BRCA1 and BRCA2 confer a high risk of breast and ovarian cancer 1 , but account for only a small fraction of breast cancer susceptibility 1,2 . To find additional genes conferring susceptibility to breast cancer, we analyzed CHEK2 (also known as CHK2), which encodes a cell-cycle checkpoint kinase that is implicated in DNA repair processes involving BRCA1 and p53 (refs 3-5). We show that CHEK2*1100delC, a truncating variant that abrogates the kinase activity 6 , has a frequency of 1.1% in healthy individuals. However, this variant is present in 5.1% of individuals with breast cancer from 718 families that do not carry mutations in BRCA1 or BRCA2 (P=0.00000003), including 13.5% of individuals from families with male breast cancer (P=0.00015). We estimate that the CHEK2*1100delC variant results in an approximately twofold increase of breast cancer risk in women and a tenfold increase of risk in men. By contrast, the variant confers no increased cancer risk in carriers of BRCA1 or BRCA2 mutations. This suggests that the biological mechanisms underlying the elevated risk of breast cancer in CHEK2 mutation carriers are already subverted in carriers of BRCA1 or BRCA2 mutations, which is consistent with participation of the encoded proteins in the same pathway.

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It is possible to identify young women at high risk for breast cancer. The number of cancers detected was significantly greater than expected in an age-matched average-risk population and related to the risk category. Overall, screening parameters were comparable to population screening data, with less favorable results in the youngest age group (< 40) and BRCA1/2 carriers.