Summary If genetic testing for breast and ovarian cancer predisposition is to become available within a public health care system there needs to be a rational and cost-effective approach to mutation analysis. We have screened for BRCA 1 mutations in 230 women with breast cancer, all from the Wessex region of southern England, in order to establish the parameters on which to base a cost-effective regional mutation analysis strategy. Truncating mutations were detected in 10/155 (6.5%) consecutive cases selected only for diagnosis under the age of 40 (nine of these ten women had a strong family history of breast or ovarian cancer), 3/61 (4.9%) bilateral-breast cancer cases (all three mutations occurring among women for whom the first cancer was diagnosed under 40 years) and 8/30 (26.6%) breast cancer cases presenting to the genetics clinic (for whom a strong family history of breast and/or ovarian cancer was present). Ten different mutations were detected in 17 families, but three of these accounted for 10/17 (59%) of the families. The cost of screening the population for mutations in the entire BRCA 1 gene is unacceptably high. However, the cost of screening a carefully selected patient cohort is low, the risk of misinterpretation much less and the potential clinical benefits clearer.Keywords: BRCA 1; breast cancer; ovarian cancer; mutation analysisWith the cloning of BRCAI in 1994 (Miki et al, 1994) and BRCA2 in 1995(Wooster et al, 1995 it is now possible to screen breast or ovarian cancer patients for the presence of germline mutations. Together these genes account for about 75% of families with a highly penetrant dominantly inherited breast and/or ovarian cancer family history. There is at least one other highly penetrant breast cancer predisposition gene to be discovered (Rebbeck et al, 1997;Schubert et al, 1997;Serova et al, 1997) and undoubtedly many others of lower penetrance. Mutations in BRCAJ and BRCA2 are scattered throughout these very large genes and current methods of mutation analysis are far from perfect. Most of the available techniques detect only about 70-80% of causative mutations. A negative result from a mutation screen, in most circumstances, means very little in terms of altering the breast cancer risk to other relatives (Eeles, 1996; Healy, 1997). In families with a striking occurrence of apparently dominantly inherited breast cancer, however, the finding of a mutation can greatly refine the prediction of cancer risk and help to inform clinician and patient alike when making decisions about, for example, preventive surgery or cancer treatment.Mutation analysis is already available in a few genetics centres in the UK as part of a comprehensive molecular genetics service. As referrals for advice about inherited cancer risks escalate so do expectations for genetic testing to be provided. A rational approach to such testing is needed. We have reviewed three clinically selected groups for whom testing might be indicated and performed a comprehensive screen for mutations in BRCA 1. Using these ...