Using an approach known as RNA interference, we have specifically depleted human cells of the FANCJ DNA helicase and characterized the sensitivity of the cells to a small molecule compound that stabilizes G-quadruplex DNA structures. This work enabled us to elucidate a novel functionof the FANCJ helicase in the manintenace of chromosomal stability. Since individuals carrying homozygous mutations in the FANCJ helicase gene have a genetic disorder known as Fanconi Anemia characterized by genomic instability and cancer, we believe our results shed new insights to the cellular pathways of FANCJ that serve to counter replciational stress due to alterante DNA structures such as G-quadruplexes that arise in vivo. We have also employed yeast as a model genetic system to study the role of the Werner syndrome helicase in DNA replciation and repair. This work has enabled us to characterize the catalytic requirements of WRN in a defined genetic DNA repair pathway that operates to provide cellular resistance to alkylating agents which impose replicational stress. Lastly, we have utilized mouse as a model genetic system to characterize the role of a helicase ortholog (RECQ1) found in humans whose biological significance is not well understood. Characterization of th eprimary mouse embryonic fibroblasts from RECQ1 knockout mice has revealed that the RECQ1 helicase has unique and important roles in genomic stability maintenance.