Human herpesvirus-6 (HHV-6) and human cytomegalovirus (HCMV) have been suggested to be associated with some human malignancies. Increasing evidences suggest the involvement of HHV-6 in various lymphoproliferative disorders and oral carcinomas. Oncogenic potential of herpesviruses is of significant concern in the proposed use of live herpesvirus vaccines. Therefore, we studied the oncogenic potential and properties of HHV-6 and HCMV to evaluate their role in human cancers. Two transforming domains have been identified and mapped in the genome of CMV by introducing CMV DNA clones into rodent fibroblasts and human keratinocytes and assaying the cells for tumor growth in nude mice. One of the transforming domains has been mapped to a 79-amino acid open reading frame (ORF) and the other to two ORFs of 270-amino acid and 89-amino-acid peptides. Disruption of these ORFs by restriction digestion caused these DNAs inactive in transformation suggesting that these ORFs are essential for transforming function of CMV. The immediate early gene of CMV, a candidate subunit vaccine, has been found non-tumorigenic. Using a similar experimental approach, two transforming regions have been identified in the genome of HHV-6. One of the regions, called ZVH14 (8,7 KB), has been detected in various Hodgkin's and non-Hodgkin's lymphomas and oral carcinomas. Genomic DNAs of both A and B type strains of HHV-6 have been found to be tumorigenic in mice. To map the transforming domain and to understand the mechanism of transformation, deletion clones from ZVH14 were generated and studied. The ZVH14 DNA and a subclone potentially encoding a 115 amino acid-protein could activate transcription of a reporter gene linked to a human or mouse Harvey ras promoters. These data suggest that HHV-6 transforming genes may act through transcriptional activation of some cellular genes or oncogenes.