Exonuclease I (Exo I) from Schizosaccharomyces pombe, a 5'-->3' double-stranded DNA exonuclease, is induced during meiotic prophase I. The exo1 gene is a member of a family of related DNA repair genes, including RAD2/rad13/xpgc and YKL510/rad2, conserved from yeast to humans. An exo1 mutant displays a mutator phenotype and alters activity of the ade6-M387 marker effect. These results suggest that Exo I acts in a pathway that corrects mismatched base pairs.