DNA mismatch repair system exists in nearly all originisms. They have different components and mechanisms from prokaryote E.coli to eukaryote and human. Defects in human mismatch repair genes cause genome instability and hereditary nonpolyposis colon cancer (HNPCC) and others tumors. MutS in E.coli MMR system can specifically recognize mispaired or unpaired base and has been developed into powerful tools for detecting gene mutations and polymorphisms.