BMC Cancer 2007, 7:218 http://www.biomedcentral.com/1471-2407/7/218 IFmigmuurneo3histochemical aspects of β-catenin antigen stain; original magnification, 40× Immunohistochemical aspects of β-catenin antigen stain; original magnification, 40×. (A) Pleomorphic adenoma in human sali- vary gland with membrane and cytoplasmic β-catenin stain; (B) Mixed tumour in canine mammary tumour with membrane and cytoplasmic β-catenin stain; (C) Carcinoma ex-pleomorphic adenoma in human salivary gland showing β-catenin nuclear stain (arrows); (D) Metaplastic carcinoma in canine mammary gland showing β-catenin nuclear stain (arrows). antigenic expressions (for cytokeratins, vimentin, β-cat- enin, and E-cadherin) were identified between the lesions, suggesting common pathogenetic mechanisms in the his- togenesis of these tumours. Vimentin and the pool of cytokeratins detected by the monoclonal antibody AE1/AE3 depicted parallel immu- nolocalization pattern, confirming the utility of this anti- gen to biologically characterize tumoural components. p63 immunolocalization was restricted to cells with myoepithelial morphological features in PA, and to lesions of myoepithelial differentiation in Ca ex-PA, albeit the less intense staining in the latter suggests some loss of differentiation [44]. Similar observations were found for canine tumours, corroborating the use of this marker to demonstrate myoepithelial cells in both species. How- ever, the analysis of the antigenic behaviour in tumours of both species is hindered by the fact that p63 possesses two different isoforms (TAp63 and ΔNp63) with opposite functions, being responsible for cell-cycle arrest and cell proliferation, respectively [45]. Clone 4A4, used in our study, recognizes all isoforms. It remains to be further evaluated by future works. The expression of cell adhesion relating β-catenin and E- cadherin proteins was also similar in both species. In PA and canine MT, β-catenin and E-cadherin presented with predominantly membrane expression. β-catenin expres- sion in MC and Ca ex-PA was either cytoplasmic/nuclear, or only nuclear, suggesting that changes in antigenic loca- tion may be related to the induction of gene transcription linked to cell proliferation in malignant tumours [34,35,45]. In addition, the loss of β-catenin and E-cad- herin membrane expression may be associated with more aggressive tumour characteristics such as invasiveness and metastasis [46-49]. The most outstanding difference in antigenic expression was related to estrogen receptor. ER immunostaining was observed in all canine lesions, and it was not detected in any human neoplasia evaluated. Several previous studies showed the presence of ER in canine mammary tumours, suggesting this protein participates in lesion formation [50-52]. The lack of immunolocalization in salivary gland Page 6 of 9 (page number not for citation purposes)