PURPOSE: Human herpesviruses have been associated with the etiology of several human cancers. The role of these viruses in carcinogenesis has not yet been clarified. This study focused on identifying and characterizing the transforming potential of cloned DNA fragments from human cytomegalovirus (HCMV). MATERIALS AND METHODS: Multiple DNA fragments of HCMV were applied to cells for transformation. Morphological transforming region II (mtrII) of HCMV strain Towne has been identified to a 3.0kb XbaI-BamHI DNA fragment which was retained in transformed cells. The transforming activity was induced by a 980 bp BaII-Xho I subfragment (pBS980) containing both promoter/ regulatory elements as well as three open reading frames (ORFs), i.e., 79ORF, 83ORF, and 34ORF. The ORFs have been evaluated for transforming potential in NIH3T3 cells. RESULTS: MtrII (pBS980) has BglII restriction enzyme site which divides into two subfragments, pBS440 and pBS540, the latter has whole 83ORF, 34ORF, and fragment of 79ORF, the former has only fragment of 79ORF. Among three ORFs, 83ORF and 34ORF were not functional in transformation, because in pBS540 these ORFs were not truncated. CONCLUSION: The 79ORF (79-aa transforming peptide) has allowed a better approach to determine the role of HCMV in human carcinogenesis.