DNA polymerase beta (polbeta) is one of mammalian DNA polymerases and is known to be involved in a G:T/G:U mismatch repair. In order to investigate an involvement of this enzyme in a base excision repair, we searched a mutation of human polbeta in human gastric cancer and studied a function of the mutation. We observed cancer-specific missense mutations in 6 of 20 samples. All of these mutations were, however, heterozygous. We further analyzed the base excision repair activity of these mutants to know whether these mutants cause an error of mismatch repair. One of these mutants, which resulted in an amino acid substitution of Glu for Lys at codon 295, showed an inhibitory effect by in vitro base excision repair assay, suggesting that this mutation might play some role in carcinogenesis of the gastric mucosa.