MSH2 and cancer: Exo1 was the first excision nuclease implicated in Msh2-dependent MMR (15, 53); however, deletion of EXO1 causes only a small MMR defect in Saccharomyces cerevisiae that is less that 1% of the MMR defect caused by deletion of MSH2, and Exo1−/− knockout mice have far less severe cancer phenotypes than Msh2−/− or Mlh1−/− mice (15, 41, 60).