Since the discovery of the first E. coli mutator gene, mutT, most of the mutations inducing elevated spontaneous mutation rates could be clearly attributed to defects in DNA repair. MutT turned out to be a pyrophosphohydrolase hydrolyzing 8-oxodGTP, thus preventing its incorporation into DNA and suppresing the occurrence of spontaneous AT-->CG transversions. Most of the bacterial mutator genes appeared to be evolutionarily conserved, and scientists were continuously searching for contribution of DNA repair deficiency in human diseases, especially carcinogenesis. Yet a human MutT homologue--hMTH1 protein--was found to be overexpressed rather than inactivated in many human diseases, including cancer. The interest in DNA repair contribution to human diseases exploded with the observation that germline mutations in mismatch repair (MMR) genes predispose to hereditary non-polyposis colorectal cancer (HNPCC). Despite our continuously growing knowledge about DNA repair we still do not fully understand how the mutator phenotype contributes to specific forms of human diseases.