Biomolecular evidence has shown that ductal carcinoma in situ (DCIS) may develop into invasive carcinoma of the canine mammary gland, and mutations in proto-oncogenes HER2 and EGFR; two members of the family of epidermal growth factor receptors, may be involved in this process. The purpose of this study was the characterization of the immunohistochemical expression of the EGFR and HER2 proteins in the process of neoplastic transformation, supposedly present in ductal carcinomas in situ in canine mammary glands. Fifteen cases of DCIS were evaluated, with a higher expression of HER2 and EGFR being observed in low-grade carcinomas when compared with high-grade neoplasms, and with a high positive statistical correlation in the latter. Results suggest that aggressive tumors tend to lose the expression of EGFR and HER2 simultaneously. The loss of the expression of these markers may be related to the process of neoplastic progression in canine mammary tumors. Keywords: dog, neoplasm, immunohistochemistry, EGFR, CerbB2 RESUMO Evidências biomoleculares sugerem que o carcinoma ductal in situ (CDIS) pode progredir para carcinoma invasor na mama canina e que mutações nos proto-oncogenes HER-2 e EGFR, dois membros da família de receptores para fatores de crescimento epidérmicos, podem estar envolvidas neste processo. A partir disso, este trabalho teve por objetivo caracterizar a expressão imuno-histoquímica das proteínas EGFR e HER-

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