The long term goal of this project is to determine the mechanism of recombination of virus DNA with the host chromosome. Recombination between virus and host involves specialized regions in each DNA called attachment (att) sites. We have established that recombination between att sites is promoted by a direct interaction between identical nucleotides in a specific sub-region of each site. We have devised a novel way to isolate many different mutations of this sub-region, and we are also using standard techniques to isolate mutants outside of it. We have determined the sequence of a new secondary attachment site and can account for its recombinogenicity. Finally, we have characterized an enzyme that specifically cleaves branched DNA; such an enzyme is thought to have a key role in recombination.