Mismatch repair genes are involved in increasing the fidelity of replication by specific repair of DNA polymerase incorporation errors. In Escherichia coli, the best studied mismatch repair (MMR) pathway is the methyl-directed long patch repair system which is mediated by three gene products; MutS, MutL and MutH. These are conserved in higher eukaryotes. Mutations in human homologues of these proteins have been shown to be implicated in hereditary non-polyposis colorectal cancer (HNPCC). Alterations in the coding regions of MMR genes result in a mutator phenotype with marked instability of microsatellite sequences, indicative of a deficiency in DNA repair.