Abstract Multiple genetic changes are necessary to convert a normal cell into a cancer cell. With a few exceptions, the mutations or conditions (for example, DNA replication stress) that initiate genetic instability and tumor formation are not understood. The goal of the proposed research is to use the yeast Saccharomyces cerevisiae as a model for investigating the mechanisms that generate the various types of genetic instability related to carcinogenesis. As an example of the success of this approach, our demonstration that DNA mismatch repair mutations in yeast produced the same rate of microsatellite instability observed in hereditary colorectal cancer cells was an important clue as to the causal mutation. 1. One of the proposed studies is the characterization of a novel mutator that is responsive to base composition. We showed that a gene with high GC content had a much-elevated mutation rate than genes with lower GC contents. We subsequently identified a mutation (met18) that elevated the mutation rate of a high-GC gene more than 100-fold, with a much smaller effect on other genes. The Met18 protein is a chaperone involved in the transport of Fe-S clusters into a variety of enzymes involved in DNA replication and DNA repair. Our current data suggest that DNA polymerase delta lacking the Fe-S cluster has substantially reduced processivity, accounting for the mutator phenotype of met18. This hypothesis will be tested by our proposed experiments. 2. Fusions between centromeres are commonly observed in tumor cells. We have developed a genetic system that allows the identification of recombination events occurring between the centromeres of different yeast chromosomes. We will use this system to examine the genetic regulation of centromere-centromere recombination. 3. We have recently used the mammalian APOBEC protein, which deaminates cytosine in single-stranded DNA, to map regions of single- stranded DNA in yeast cells undergoing DNA replication stress. Using APOBEC-induced mutations, we propose extending our studies to map deletions and duplications in arrays of tandemly-repeated genes. Such alterations are an important cause of genome instability. 4. Lastly, the Tel1p and Mec1p yeast proteins are related functionally to the human ATM and ATR proteins, respectively. Mutations in both human genes are associated with certain classes of tumors. Yeast cells with tel1 mec1 mutations have greatly elevated rates of chromosome rearrangements and chromosome non-disjunction. The chromosome rearrangements, but not the aneuploidy, are a consequence of a high rate of telomere fusions. We are testing the hypothesis that the high level of chromosome non-disjunction is a consequence of defective phosphorylation of histone H2A in tel1 mec1 strains.