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or mixed-type cells. CMTs can be benign or malignant, with about 50 % of the tumors being benign. Generally, complex adenoma and benign mixed tumor are the dominant types, whereas pure benign mesenchymal tumors are rare (Hellmén et al., 2000). Because CMTs and human mammary cancers share similar properties in biologi-cal behavior, histology and epidemiology (MacEwen, 1990; Martin et al., 1984; Misdorp and Hart, 1976; Nerurkar et al., 1989; Owen, cells caused by excessive expression of hormone receptors, insufficient apoptosis or dysregulation of cellular differentiation (Hanahan and Weinberg, 2000). In recent years, markers associ-ated with the genetic or epigenetic alternations in tumor cells have become a useful tool in the categorization, invasiveness, prognosis and chemoresistance of canine and human mammary cancers (Hellmén et al., 1988, 1993; Hellmén and Lindgren, 1989). In the canine, markers such as p63, vimentin and cytokerations have been shown to be specific to different mammary cell types. Both p63 and vimentin are markers of myoepithelial cells (Gama et al., 2003), whereas cytokeratin has been used as a sensitive marke