The qin oncogene is a cell-derived insert in the genome of avian sarcoma virus 31 (ASV 31) and functions as the oncogenic determinant of that virus. Overexpression of the viral and cellular versions of the Qin protein (v-Qin and c-Qin) induces oncogenic transformation of chicken embryo fibroblasts (CEF); v-Qin also rapidly induces fibrosarcomas in chickens. Qin proteins can bind to specific DNA sequences and act as transcriptional repressors. In this study, mutants of Qin were constructed in order to determine the molecular domains required for transformation of chicken embryo fibroblasts. Our data indicate that three regions required for transforming activity are located (i) between residues 74-141 at the amino terminus, (ii) in the winged helix domain and (iii) between residues 383-395 at the carboxyl terminus. A Qin mutant with 12 amino acids deleted from the carboxyl terminus (383-395) showed transforming activity that was lower than that of wild type Qin for CEF. Compare to wild type Qin transformants, the mutant transformed cells had a reduced ability for multilayered and for anchorage independent growth. Deletion of 48 amino acids from the carboxyl terminus of the Qin protein (347-395) completely abolished transforming activity. In contrast, deletion of 74 amino acids from the amino terminus did not affect transformation of CEF. However, further deletion of 68 amino acids (74-141) reduced but did not abolish transforming activity. Finally, deletion in the winged helix domain (218-295) completely abrogated oncogenic capacity in CEF. These results suggest that DNA binding and transcriptional repression may be important in Qin-induced oncogenic transformation.