Biochemical assay for mismatch correction. The substrate for mismatch repair is a covalently closed heteroduplex of f1R229 containing a mismatch within the EcoRI site. Methyl groups indicate the locations of the four GATC sites within the DNA. Cleavage of mismatch heteroduplexes with EcoRI and BamHI yields just the full-length linear BamHI product since the hybrid EcoRI site is resistant to cutting. Mismatch correction on the strand containing the mutant EcoRI sequence renders the site sensitive to cleavage, and now treatment with both restriction endonucleases yields two fragments as indicated.