Adenovirus genetic research has entered a new phase with the recent upsurge in interest in using the virus as a vector for gene transfer, gene therapy, and as a vaccine (reviewed in refs. 1–3). Although the idea of using adenovirus as a vector is not new (4), recent advances both in understanding the functions of many of the early genes, and in the ability to manipulate the genome to create the desired genotype, have made it possible to tailor the desired recombinant to very specific purposes. Less attention has been paid to the possibility of creating adenovirus vectors that express proteins that have direct effects on the genome of the vector itself, or on coinfecting adenovirus genomes. Examples of the types of investigation that could be conducted into such trans effects include: