DNA oxidation is the process of oxidative damage of deoxyribonucleic acid. As described in detail by Burrows et al., 8-oxo-2'-deoxyguanosine (8-oxo-dG) is the most common oxidative lesion observed in duplex DNA because guanine has a lower one-electron reduction potential than the other nucleosides in DNA. The one electron reduction potentials of the nucleosides (in volts versus NHE) are guanine 1.29, adenine 1.42, cytosine 1.6 and thymine 1.7. About 1 in 40,000 guanines in the genome are present as 8-oxo-dG under normal conditions. This means that >30,000 8-oxo-dGs may exist at any given time in the genome of a human cell. Another product of DNA oxidation is 8-oxo-dA. 8-oxo-dA occurs at about 1/10 the frequency of 8-oxo-dG. The reduction potential of guanine may be reduced by as much as 50%, depending on the particular neighboring nucleosides stacked next to it within DNA. Excess DNA oxidation is linked to certain diseases and cancers, while normal levels of oxidized nucleotides, due to normal levels of ROS, may be necessary for memory and learning. Oxidized bases in DNA More than 20 oxidatively damaged DNA base lesions were identified in 2003 by Cooke et al. and these overlap the 12 oxidized bases reported in 1992 by Dizdaroglu. Two of the most frequently oxidized bases found by Dizdaroglu after ionizing radiation (causing oxidative stress) were the two oxidation products of guanine shown in the figure. One of these products was 8-OH-Gua (8-hydroxyguanine). (The article 8-oxo-2'-deoxyguanosine refers to the same damaged base since the keto form 8-oxo-Gua described there may undergo a tautomeric shift to the enol form 8-OH-Gua shown here.) The other product was FapyGua (2,6-diamino-4-hydroxy-5-formamidopyrimidine). Another frequent oxidation product was 5-OH-Hyd (5-hydroxyhydantoin) derived from cytosine. Removal of oxidized bases Most oxidized bases are removed from DNA by enzymes operating within the base excision repair pathway. Removal of oxidized bases in DNA is fairly rapid. For example, 8-oxo-dG was increased 10-fold in the livers of mice subjected to ionizing radiation, but the excess 8-oxo-dG was removed with a half-life of 11 minutes. Steady-state levels of DNA damages Steady-state levels of endogenous DNA damages represent the balance between formation and repair. Swenberg et al. measured average amounts of steady state endogenous DNA damages in mammalian cells. The seven most common damages they found are shown in Table 1. Only one directly oxidized base, 8-hydroxyguanine, at about 2,400 8-OH-G per cell, was among the most frequent DNA damages