Editor—Germline mutations in theBRCA1 and BRCA2genes cause predisposition to breast and ovarian cancer.1Epidemiological evidence and linkage studies suggested that the likelihood that a woman with breast cancer has a genetic susceptibility to the condition is greater the younger she was at diagnosis and with increasing extent of family history of the disease. Studies of the prevalence of germline mutations in BRCA1and BRCA2 in women with breast cancer has enabled the frequency of mutations to be determined in women with different ages at diagnosis and extent of family history of breast cancer.2 3 The CASH study into the attributable risk of breast and ovarian cancer estimated that 33% of all breast cancers diagnosed by the age of 29 years, and 22% diagnosed by the age of 30-39 years, are the result of an inherited mutation.4However, the proportion of breast cancer cases diagnosed by 40 years resulting from a BRCA1 mutation was predicted to be 5.3%.5 Previous population based studies of the prevalence of BRCA1 mutations in early onset breast cancer have been in cases unselected for family history, and the majority of mutation carriers detected did have some degree of family history of either breast or ovarian cancer.6-10 The aim of this study was to establish the prevalence of BRCA1 mutations in a large series of British patients with a young age of onset and no known family history of the disease, since such patients are referred relatively frequently for genetic counselling. The presence ofBRCA1 mutations in a significant proportion of these patients would have important implications for the planning of a mutation screening strategy in diagnostic services.

Patients were ascertained from the Imperial …