Oxygen radicals have been suggested to be involved in mutation/carcinogenesis. The C-8 position of guanine residues in DNA is hydroxylated to produce 8-hydroxyguanine (8-OH-Gua) in DNA in vitro by various oxygen radical producing agents. The formation of 8-OH-Gua was also observed in cellular DNA in vivo by radiation or oxygen radical forming carcinogens. The 8-OH-Gua residue in DNA is often misread in the position of 8-OH-Gua residue itself but also at neighboring residues next to 8-OH-Gua. When second guanine in codon 12 was specifically replaced with 8-OH-Gua and transferred to NIH3T3, the recipient cells were transformed to malignant cell type. E. coli was found to contain an endonuclease which specifically recognizes 8-OH-Gua residue and cleave DNA strand before and after the modified base. The data obtained imply that 8-OH-Gua formed in DNA in vivo is recognized as an abnormal modified base which, if not repaired, play a role in the mediation of oxygen radical-involved mutation/carcinogenesis.