The SGS1 of Saccharomyces cerevisiae is a homologue of the Bloom's syndrome and Werner's syndrome genes. The sgs1 disruptants show hyperrecombination, higher sensitivity to methyl methanesulfonate and hydroxyurea, and poor sporulation. In this study, we found that sister chromatid exchange was increased in sgs1 disruptants. We made mutated SGS1 genes coding a protein proved to lack DNA helicase activity (sgs1-hd), having equivalent missense mutations found in Bloom's syndrome patients (sgs1-BS1, sgs1-BS2). None of the mutated genes could suppress the higher sensitivity to methyl methanesulfonate and hydroxyurea and the increased frequency of interchromosomal recombination and sister chromatid exchange of sgs1 disruptants. On the other hand, all of the mutant genes were able to complement the poor sporulation phenotype of sgs1 disruptants, although the values were not as high as that of wild-type SGS1.