with topoisomerase III at the N-terminal region is involved in the suppression of spontaneous and damage induced recombination and the absence of this interaction results in a lethal or very severe phenotype. The emerging picture clearly is that RecQ helicases in concert with Top 3 are involved in maintaining genomic stability and integrity by controlling recombination events, and repairing DNA damage in the G2-phase of the cell cycle. The importance of RecQ for genomic integrity is exemplified by the diseases that arise as a consequence of mutations or malfunctions in RecQ helicases; thus it is crucial that RecQ is present and functional to ensure proper human growth and development. WRN helicase The Werner syndrome ATP-dependent helicase (WRN helicase) is unusual among RecQ DNA family helicases in having an additional exonuclease activity. WRN interacts with DNA-PKcs and the Ku protein complex. This observation, combined with evidence that WRN deficient cells produce extensive deletions at sites of joining of non-homologous DNA ends, suggests a role for WRN protein in the DNA repair process of non-homologous end joining (NHEJ). WRN also physically interacts with the major NHEJ factor X4L4 (XRCC4-DNA ligase 4 complex). X4L4 stimulates WRN exonuclease activity that likely facilitates DNA end processing prior to final ligation by X4L4. WRN also appears to play a role in resolving recombination intermediate structures during homologous recombinational repair (HRR) of DNA double-strand breaks. WRN participates in a complex with RAD51, RAD54, RAD54B and ATR proteins in carrying out the recombination step during inter-strand DNA cross-link repair. Evidence was presented that WRN plays a direct role in the repair of methylation induced DNA damage. The process likely involves the helicase and exonuclease activities of WRN that operate together with DNA polymerase beta in long patch base excision repair. WRN was found to have a specific role in preventing or repairing DNA damages resulting from chronic oxidative stress, particularly in slowly replicating cells. This finding suggested that WRN may be important in dealing with oxidative DNA damages that underlie normal aging (see DNA damage theory of aging). BLM helicase Cells from humans with Bloom syndrome are sensitive to DNA damaging agents such as UV and methyl methanesulfonate indicating deficient DNA repair capability. The budding yeast Saccharomyces cerevisiae encodes an ortholog of the Bloom syndrome (BLM) protein that is designated Sgs1 (Small growth suppressor 1). Sgs1(BLM) is a helicase that functions in homologous recombinational repair of DNA double-strand