The objective of this research is to continue the development and application of a model system for investigating the genetic effects of specific forms of chemical damage to the genome. In the work done over the past five years, we developed a strategy for building 06-methylguanine, as representative of a model carcinogen-DNA adduct, into a unique site in the genome of a bacterial virus. This adduct was replicated in vivo, and the qualitative and quantitative features of its mutagenesis were defined, as well as factors that influenced its repair. We have preliminary evidence that the strategy we have developed is of more general value for building into DNA other carcinogen-DNA adducts, even adducts that are bulky and/or chemically labile. In this application we are proposing to investigate the genetic effects of several of the DNA lesions formed by carcinogenic alkylating agents and by the potent liver carcinogen, aflatoxin B1.