Among other pathways recent studies have indicated the HIPPO signalling as an important key regulator of both physiological and neoplastic cell growth and differentiation. Particularly, the HIPPO kinases phosphorilation cascade controls cell growth, proliferation and survival and also divergent physiological effects such as cell differentiation, stem cell renewal, reprogramming, and patterning. Recently, HYPPO signalling has been shown to correlate with high grade and malignancy and with cancer stem cell content in human breast cancer (HBC). Particularly, the activity of TAZ, a transducer of the Hippo pathway, has been found to sustain self-renewal and tumor-initiation capacities of breast stem cells. In addition, the biological activity of TAZ in breast cancer presents also intriguing analogies with epithelial-mesenchymal transition (EMT) since TAZ was found to be increased during EMT in HBC. Here we investigated TAZ expression in canine (CMTs) and feline mammary tumors (FMTs), with particular regards to the basal/myoepithelial cells diversity typically distinguishing these tumors in the two species. Sixty carcinomas were assessed by immunohistochemistry. TAZ was found to be diffusely overexpressed in grade III simple carcinomas both in the dog and the cat. In addition basal cells were TAZ positive both in neoplastic and hyperplastic canine glandular tissue. Interestingly a mild to moderate positivity was detected also in basal undifferentiated cells of feline ductal carcinomas. These preliminary results indicate that TAZ might represent an important pathway transducer for aggressiveness and might regulate basal cells proliferation and fate.