The population of Sardinia is characterized by a relatively low level of genetic heterogeneity: therefore 'founder mutations' can be expected to be found. We analysed 17 probands from families with high incidence of breast cancer or breast and ovarian cancer by sequencing the full-length coding regions of BRCA1 and BRCA2 genes. A novel BRCA2 frame-shift mutation, 3951del3insAT, which produces a protein truncated at codon 1258, was observed in six patients with BC from the same village. The mutation was not found in unaffected females (matched on basis of ethnicity and age) with no family history of cancer. Haplotype analysis strongly suggests that all affected persons had a common ancestor. The identification of this clinically significant founder mutation may facilitate screening/testing for inherited risk of breast cancer.