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The BRCA/RAD51 complex of tumor suppressor genes plays a major role in the DNA damage response. In this explorative study, BRCA1, BRCA2, and RAD51 mRNA expression was quantified in highly defined laser microdissected tissue samples of simple adenomas, adenocarcinomas of the mammary gland, and their lymph node metastases by real-time quantitative reverse transcription polymerase chain reaction. Expression levels in the tumors were normalized to the geometric mean of 3 housekeeping genes and quantified relative to normal mammary epithelium of the same dog. In adenomas, mRNA expression was reduced for BRCA1 (6/10 dogs, 60%), BRCA2 (4/10 dogs, 40%), and RAD51 (4/10, 40%). In adenocarcinomas BRCA1 expression varied with increased expression in 3 of 10 (30%) dogs and no differences in 7 of 10 (70%) dogs when compared with normal mammary gland. BRCA2 and RAD51 were overexpressed in 5 of 10 (50%) and 6 of 10 (60%) of adenocarcinomas, respectively. An overexpression of RAD51 and BRCA2 was found in 8 of 10 (80%) and 5 of 10 (50%) of the lymph node metastases, respectively. Direct comparison of primary tumors and metastases revealed increased mRNA expression of BRCA1 (2/10 dogs, 20%), BRCA2 (2/10 dogs, 20%), and RAD51 (3/10 dogs, 30%) in lymph node metastases. Taken together, the results suggest that RAD51 is upregulated in the majority of lymph node metastases of canine mammary tumors. Further experimental studies are needed to clarify whether these changes in gene expression are a direct carcinogenetic stimulus or a protective response due to genetic instability during tumor progression.