Mammary tumors are the most common neoplasm in female dogs, Canis canis, and in women. Mutations in human Brca2 confer an increased risk of female breast cancer. Previous studies have shown that the Brca2 tumor suppressor protein interacts with the recombinational repair protein Rad51. We cloned the full-length cDNA of the canine homologues of Brca2 and Rad51 to obtain a basis for studying their relationship with susceptibility to mammary tumors. The canine Brca2 and Rad51 cDNAs are 11 and 1.5 kb long, encoding 3.471 and 339 amino acids, respectively. The amino acid sequence of canine Brca2 showed 68% homology with the human protein, and 58% homology with a murine protein. There were highly conserved regions in the C-terminus of all three proteins, where the Rad51 interacting domain and putative nuclear localization signals are located. Comparing with the partial genomic sequence previously reported, we found possible nuclear polymorphisms in exon 11, some of which result in amino acid substitutions. On the other hand, canine Rad51 protein had extremely high homology (99%) to the human and murine proteins. Expression of both Brca2 and Rad51 was detected in the mammary gland, suggesting that these two genes interact in the canine mammary gland.