AbstractPolyomavirus (Py) encodes a potent oncogene, the middle T antigen (MT), that induces cell transformation by binding to and activating several cytoplasmic proteins which take part in transduction of growth factors‐induced mitogenic signal to the nucleus. We have previously reported that the AP‐1 transcriptional complex is a target for MT during cell transformation although, its activation was not sufficient for establishment of the transformed phenotype. Here we show that expression of a dominant‐negative cJun mutant in MT transformed cell lines inhibits its transformation ability, indicating that constitutive AP‐1 activity is necessary for cell transformation mediated by MT. Evidences also suggest that proliferation of MT transformed cells in low serum concentrations and their ability to form colonies in agarose are controlled by distinct mechanisms. J. Cell. Biochem. 90: 253–266, 2003. © 2003 Wiley‐Liss, Inc.