A human hMutT2 polypeptide and DNA (RNA) encoding such polypeptide and a procedure for producing such polypeptide by recombinant techniques is disclosed. Also disclosed are methods for utilizing such polypeptide for hydrolyzing and eliminating oxidized guanine nucleotides from the nucleotide pool to ensure correct DNA synthesis. Diagnostic assays are also disclosed which detect the presence of a mutated form of hMutT2 and over-expression of the hMutT2 protein.