Inherited mutations of the BRCA1 and BRCA2 genes, whose protein products are necessary for the homology-directed DNA repair pathway, confer a dominant susceptibility to cancer. We have investigated whether mutations of genes encoding other components of the same DNA repair pathway can also affect cancer susceptibility. We have identified three novel non-synonymous substitutions in one such gene, encoding the RAD51-related protein XRCC3. One of these variants, D213N, occurs in a highly conserved ATP-binding domain and completely abrogates the ability of the transfected gene to correct the phenotype of XRCC3 deficient cells. The D213N variant was found in the heterozygous state in DNA from 3/1577 healthy individuals. However, we did not detect this variant at all amongst 187 breast cancer families and 1300 unrelated patients with common cancers. Thus we have no evidence that D213N increases the risk of cancer. We propose that not all components of the homologous recombination repair complex can act as cancer susceptibility genes.