The pathways leading to G:C-->C:G transversions and their repair mechanisms remain uncertain. CIC and GIG mismatches arising during DNA replication are a potential source of G:C-->C:G transversions. The Escherichia coli mutHLS mismatch repair pathway efficiently corrects G/G mismatches, whereas CIC mismatches are a poor substrate. Escherichia coli must have a more specific repair pathway to correct CIC mismatches. In this study, we performed gel-shift assays to identify CIC mismatch-binding proteins in cell extracts of E,coli. By testing heteroduplex DNA (34mers) containing CIC mismatches, two specific band shifts were generated in the gels The band shifts were due to mismatch-specific binding of proteins present in the extracts. Cell extracts of a mutant strain defective in MutM protein did not produce a low-mobility complex, Purified MutM protein bound efficiently to the C/C mismatch-containing heteroduplex to produce the low-mobility complex. The second protein, which produced a high-mobility complex with the CIC mismatches, was purified to homogeneity, and the amino acid sequence revealed that this protein was the FabA protein of E.coli. The high-mobility complex was not formed in cell extracts of a fabA mutant. From these results it is possible that MutM and FabA proteins are components of repair pathways for C/C mismatches in E,coli, Furthermore, we found that Saccharomyces cerevisiae OGG1 protein, a functional homolog of E.coli MutM protein, could specifically bind to the CIC mismatches in DNA.