Increased spontaneous mutation frequency is associated with increased cancer risk. However, the relative contribution of spontaneous endogenous mutagenesis to carcinogenesis is not known today. Defects in the postreplication DNA mismatch repair (MMR) pathway are recognized to increase spontaneous mutations. Mutations in MMR genes cause hereditary non-polyposis colon cancer. In an effort to identify unidentified genes involved in MMR and tissue-specific MMRassociated factors, we employed the yeast two-hybrid system, using the human hMSH2 as bait and a human fetal liver cDNA library as prey. We demonstrated that hMSH2 interacts with a human 5’ → 3’ exonuclease 1 (hEXO1). Data presented in this thesis also support the conclusion that mitochondrial dysfunction leads to spontaneous nuclear DNA damage. We employed the yeast Saccharomyces cerevisiae as a model system to investigate a potential link between mitochondrial activity and genomic instability. Mitochondrial dysfunction and genetic instability are characteristic features of cancer cells. Furthermore, mitochondrial dysfunction is a key feature of aging due to accumulation of mutations in mtDNA. Our studies in a yeast model system suggest that mitochondria contain some intrinsic properties that control the generation of the mutator phenotype associated with cancer cells. We hypothesize that cancer cells by losing their mitochondrial function create a mutator phenotype. Given the importance of maintaining the integrity of the mitochondrial genome we have found that it might be valuable to further investigate the molecular processes and components responsible for mtDNA repair. It has recently been recognised that base excision DNA repair (BER) is operating in the mitochondria, however, knowledge about other repair pathways is still very limited. We decided to investigate O6-methylguanine- DNA methyltransferase (MGMT) because of the fact that its sub-cellular localization has not been determined. We determined that it was localized to nucleus but not to mitochondria in HeLa and breast epithelial cells.