Adenovirus (Ad) and adeno-associated virus(AAV) have attractive and complementary properties that canbe exploited for gene transfer purposes. Ad vectors are probablythe most efficient vehicles to deliver foreign genes both invitro and in vivo. AAV exhibits the unique ability to establishlatency by efficiently integrating at a specific locus of humanchromosome 19 (AAVS1). Two viral elements are necessaryfor the integration at AAVS1: Rep68y78 and the invertedterminal repeats (AAV-ITRs). In this study, we report thedevelopment of two helper-dependent adenoviral (HD) vectors,one carrying the Rep78 gene, the other an AAV-ITRflankedtransgene. Although Rep proteins have been demonstratedto interfere with Ad replication, HD Rep78 vector wassuccessfully amplified on serial passages in 293CRE4 cellswith a yield of 50–100 transducing units per cell. DNAintegration at the AAVS1 site also was demonstrated inhepatoma cells coinfected with the HD-expressing Rep78 andwith the second HD vector carrying a transgene flanked byAAV-ITRs. The high transduction efficiency, large cloningcapacity, and high titer of the HD, combined with the sitespecificintegration machinery provided by AAV-derived components,make the AdyAAV hybrid viruses a promising vehiclefor gene therapy