Breast cancer, one of the most common and deleterious of all diseases affecting women, occurs in hereditary and sporadic forms. Hereditary breast cancers are genetically heterogeneous; susceptibility is variously attributable to germline mutations in the BRCA1 (ref. 1), BRCA2 (ref. 2), TP53 (ref. 3) or ataxia telangiectasia (ATM) genes, each of which is considered to be a tumour suppressor. Recently a number of germline mutations in the BRCA2 gene have been identified in families prone to breast cancer. We screened 100 primary breast cancers from Japanese patients for BRCA2 mutations, using PCR-SSCP. We found two germline mutations and one somatic mutation in our patient group. One of the germline mutations was an insertion of an Alu element into exon 22, which resulted in alternative splicing that skipped exon 22. The presence of a 64-bp polyadenylate tract and evidence for an 8-bp target-site duplication of the inserted DNA implied that the retrotransposal insertion of a transcriptionally active Alu element caused this event. Our results indicate that somatic BRCA2 mutations, like somatic mutations in the BRCA1 gene, are very rare in primary breast cancers.