Recent studies have demonstrated novel alterations of microsatellite DNA in tumor tissue. The alterations, termed microsatellite instability or replication error phenotype, have now been observed in tumors from patients with hereditary nonpolyposis colorectal cancer (HNPCC), the Muir-Torre syndrome (MTS) and in an increasing number of sporadic tumors. These observations, along with the use of genetic linkage analysis, have led to the identification of at least four genetic susceptibility loci for HNPCC, hMSH2, hMLH1, hPMS1 and hPMS2, each of which are involved in DNA mismatch repair. For those tumors demonstrating microsatellite instability, several different phenotypes may exist, the significance of which is currently unknown. Defective DNA mismatch repair may have important implications for the mechanism of tumorigenesis and the clinical behavior of tumors.