Microsatellite instability (MSI) due to defects in mismatch repair (MMR) genes such as MLH1 and MSH2 are involved in the carcinogenesis and tumour progression of sporadic and inherited human cancers (Eshleman and Makowitz, 1996; Kinzler and Vogelstein, 1996). The gene discussed is MLH1; the disease is neoplasm.