Infection with herpes simplex virus-1 (HSV-1) can lead to diverse outcomes ranging from asymptomatic infection to life threatening encephalitis. Emergence of drug resistant viral isolates, leading to treatment failure, is caused by rapid generation of mutations. For DNA viruses (such as herpesviruses) it is assumed that recombination and complementation can provide important mechanisms for generating and maintaining genome diversity. We therefore suggest that the phenomenon (We recently identified) that only a limited number of incoming herpesvirus genomes are able to initiate expression and replication, may have a role in regulating the number of mutations. In the proposed project we will characterize this bottleneck mechanism in order to test this hypothesis. We will identify the steps in the infection process in which this bottleneck occurs. We will find host and viral proteins that regulate the number of viral genomes being expressed and replicated. Finally, we will develop methods for testing the evolutionary benefits of this bottleneck. This project will uncover new regulatory elements involved in herpesvirus replication and may, as well, reveal more general concepts in virology.