Considering the similarity in many aspects between mammary tumors in women and female dogs, this study aimed to analyze the expression of H3K9Ac, H4K12Ac, HDAC1, HDAC2 and HDAC6 in simple mammary carcinomas in dogs and to correlate the clinicopathological variables and the prognosis with the expression of these proteins. The gene discussed is HDAC2; the disease is breast cancer.