The discovery that mutations in DNA mismatch repair genes can cause hereditary nonpolyposis colorectal cancer has stimulated interest in understanding the mechanism of DNA mismatch repair in eukaryotes. In the yeast Saccharomyces cerevisiae, DNA mismatch repair requires the MSH2, MLH1, and PMS1 proteins. Experiments revealed that the yeast MLH1 and PMS1 proteins physically associate, possibly forming a heterodimer, and that MLH1 and PMS1 act in concert to bind a MSH2-heteroduplex complex containing a G-T mismatch. Thus, MSH2, MLH1, and PMS1 are likely to form a ternary complex during the initiation of eukaryotic DNA mismatch repair.