We are using the bacteriophage T4 rII system as a model to explore mechanisms of DNA damage and mutagenesis. Because traditional DNA sequencing methods for analyzing the molecular nature of rII mutations are laborious and slow, we are developing methods based on genomic sequencing. With this method, important classes of mutations will be examined for their sequence changes. For instance, even mild heat damages DNA and could, if not repaired, produce on the order of 100 mutations per diploid human cell per day. Earlier studies showed that heat induces both transitions and transversions at G:C base pairs in phage T4. Genetic studies suggested that the main heat-induced transversion pathway is G:C to C:G but did not exclude G:C to T:A. We have shown that the actual mispairing of heat-damaged guanine (G*) Is G*:A. This work has now been published and the project is completed.