The Saccharomyces cerevisiae gene SGS1 encodes a protein related to the human DNA helicases defective in Werner syndrome, Bloom syndrome and Rothmund-Thomson syndrome. Yeast cells lacking the Sgs1 helicase and a related helicase, Srs2, are inviable or grow extremely poorly. The growth defect can be rescued by crippling the homologous recombination repair pathway, indicating that the essential function of these genes is related to recombination