Alterations of the human mismatch repair genes have been linked to hereditary non-polyposis colon cancer (HNPCC) as well as to sporadic cancers that exhibit microsatellite instability. The human mismatch repair genes are highly conserved homologs of the Escherichia coli MutHLS system. Six MutS homologs have been identified in Saccharomyces cerevisiae and four MutS homologs have been identified in human cells. At least three of these eukaryotic MutS homologs are involved in the recognition/binding of mispaired nucleotides and nucleotide lesions. MSH2 plays a fundamental role in mispair recognition whereas MSH3 and MSH6 appear to modify the specificity of this recognition. The redundant functions of MSH3 and MSH6 explain the greater prevalence of hmsh2 mutations in HNPCC families.