More than 75% of the reported mutations in the hereditary breast and ovarian cancer gene, BRCA1, result in truncated proteins. We have used the protein truncation test (PTT) to screen for mutations in exon 11, which encodes 61% of BRCA1. In 45 patients from breast and/or ovarian cancer families we found six novel mutations: two single nucleotide insertions, three small deletions (1-5 bp) and a nonsense mutation identified two unrelated families. Furthermore, we were able to amplify the remaining coding region by RT-PCR using lymphocyte RNA. Combined with PTT, we detected aberrantly spliced products affecting exons 5 and 6 in one of two BRCA1-linked families examined. The protein truncation test promises to become a valuable technique in detecting BRCA1 mutations.