Our goals are to determine the mechanism by which novel DNA rearrangements occur in vivo. The two model systems we employ are the integration-excision reactions of bacteriophage lambda and the formally similar genomic rearrangements of Herpes simplex. We are isolating and characterizing mutants of phage lambda that cannot carry out certain steps in integration or excision. We have used recombinant DNA technology to construct novel substrates and enzyme sources for this integration-excision system. We are developing this same technology for isolation and characterization of the DNA regions involved in Herpes simplex genomic rearrangement.