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RAD51 is a key enzyme of homologous recombination and repair of DNA double-strand breaks. RAD51 mRNA expression levels are significantly increased in laser-microdissected mammary simple carcinomas and their lymph node metastases when compared to adenomas or nonneoplastic mammary gland of the same dog. Here, RAD51 protein expression was analyzed by immunohistochemistry in paraffin-embedded mammary carcinomas and their lymph node metastases of 40 dogs, adenomas of 48 dogs, and nonneoplastic mammary gland of 88 dogs. Number of cells with nuclear RAD51 expression was significantly (P < or = .05) increased in carcinomas when compared to adenomas and metastases. In contrast, no significant differences in the number of RAD51-expressing cells were detected when metastases were compared with adenomas and nonneoplastic gland. RAD51 expression in carcinomas was correlated with expression in metastases but not with histologic grade. In conclusion, the increased number of RAD51-expressing cells in carcinomas might indicate genomic instability in these cells. Nevertheless, the increased RAD51 mRNA expression in metastases could not be confirmed by immunohistochemistry.