A change in the number of base pairs within a coding sequence can result in a frameshift mutation, which almost invariably eliminates the function of the encoded protein. A frameshift reversion assay with Saccharomyces cerevisiae that can be used to examine the types of insertions and deletions that are generated during DNA replication, as well as the editing functions that remove such replication errors, has been developed. Reversion spectra have been obtained in a wild-type strain and in strains defective for defined components of the postreplicative mismatch repair system (msh2, msh3, msh6, msh3 msh6, pms1, and mih1 mutants). Comparison of the spectra reveals that yeast mismatch repair proteins preferentially remove frameshift intermediates that arise in homopolymer tracts and indicates that some of the proteins have distinct substrate or context specificities.