The hereditary non-polyposis colorectal cancer (HNPCC)-syndrome is the most common form of hereditary colorectal cancers, and accounts for 2-7% of the total colorectal cancer burden. Since there are no single clinical features specific for HNPCC, diagnosis is based on family history (Amsterdam or Bethesda criteria) and is confirmed by the detection of a mutation in one of the responsible mismatch repair (MMR) genes. Two types of HNPCC-families can be distinguished. Type I HNPCC tumors are exclusively located in the colon, whereas in Type II HNPCC patients, extracolonic tumors are present in the stomach, endometrium, ovary, and urinary tract. The identification of the human homologues of yeast mismatch repair genes hMSH2, hMSH3, hMSH6, hMLH1, hMLH3, hPMS1 (scMLH2), and hPMS2 (scPMS1) offered the prospect of genetic screening leading to an extensive search for mutations in HNPCC-families. The majority of the alterations have been found in hMSH2 (40%) and hMLH1 (40%) genes. Mutations in the other MMR genes appear rare, absent, and/or associated with atypical families (1-5%). As a result of the mismatch repair deficiency, replication misincorporation errors accumulate, resulting in a mutator phenotype. Diagnosis of HNPCC-associated replication errors is most easily determined by the examination of a panel of the National Cancer Institute (NCI)-recommended simple repeated sequences (microsatellites), combined with immunohistochemical analysis. Although the exact molecular mechanism of the tumor development in these patients remains poorly understood, the identification of tumors that harbor a microsatellite instability has clinical and prognostic implications.