BMC Cancer 2007, 7:218 http://www.biomedcentral.com/1471-2407/7/218 IFmigmuurneo2histochemical aspects of p63 antigen stain; original magnification, 40× Immunohistochemical aspects of p63 antigen stain; original magnification, 40×. (A) Pleomorphic adenoma in human salivary gland; (B) Mixed tumour in canine mammary tumour; (C) Carcinoma ex-pleomorphic adenoma in human salivary gland; (D) Metaplastic carcinoma in canine mammary gland. Note myoepithelial p63-negative cells in malignant tumours (arrows). Analysis of E-cadherin expression has shown that benign tumours of both species presented a similar linear mem- brane expression to that of normal gland parenchyma. Malignant neoplasias depicted less intense expression, which was also related to poor cell differentiation (Figure 4). Estrogen receptor expression was not identified in tumours from the human salivary gland, while all the canine mammary tumours presented immunoreactivity for this marker (p < 0.05). Discussion The study of animal tumours may provide data to improve the understanding of similar lesions in humans, as well as tumour aetiology and development [39]. To date, there is not a universally accepted animal model for neoplastic pathology investigations. Transgenic mice, for instance, have received criticism as some of these animals are refractory to the development of certain types of tumours despite presentation of the same genetic lesion [40]. Animal cell culture is also an imperfect comparative model, as many of the events associated with carcinogen- esis variably depend on a host [41]. One of the great advantages of canine breast model is that tumours are spontaneous in this organ. As its clinical evolution is nat- ural, genetic and morphophysiological aspects may be better compared with some aspects of the human species [42]. Dogs represent a remarkable incidence of neoplasia devel- opment, usually associated with environmental exposure to important carcinogens for humans [1,43]. Tumours effecting the mammary glands, especially in females, are among the most frequent tumours observed in dogs. Finally, mixed tumours are one of the most frequent neo- plasias in dogs [19] with remarkable features in common with human salivary gland tumours, justifying the investi- gation of other possible similarities between lesions of these two species. The present study has confirmed that canine mammary gland MT and MC share some clinical characteristics with human salivary gland PA and Ca ex-PA, including age of emergence and several histopathological aspects. We have also demonstrated that commercially available antibodies for the study of human neoplasias are functional to detect antigenic expression in canine lesions. Moreover, similar Page 5 of 9 (page number not for citation purposes)