figure in this section shows a CpG site where the cytosine is methylated to form 5-methylcytosine (5mC) and the guanine is oxidized to form 8-oxo-2'-deoxyguanosine (in the figure this is shown in the tautomeric form 8-OHdG). When this structure is formed, the base excision repair enzyme OGG1 targets 8-OHdG and binds to the lesion without immediate excision. OGG1, present at a 5mCp-8-OHdG site recruits TET1, and TET1 oxidizes the 5mC adjacent to the 8-OHdG. This initiates de-methylation of 5mC. TET1 is a key enzyme involved in de-methylating 5mCpG. However, TET1 is only able to act on 5mCpG if the guanine was first oxidized to form 8-hydroxy-2'-deoxyguanosine (8-OHdG or its tautomer 8-oxo-dG), resulting in a 5mCp-8-OHdG dinucleotide (see first figure in this section). This initiates the de-methylation pathway on the methylated cytosine, finally resulting in an unmethylated cytosine, shown in the second figure in this section. Altered protein expression in neurons, due to changes in methylation of DNA, (likely controlled by 8-oxo-dG-dependent de-methylation of CpG sites in gene promoters within neuron DNA) has been established as central to memory formation. Neurological conditions Bipolar disorder Evidence that oxidative stress induced DNA damage plays a role in bipolar disorder has been reviewed by Raza et al. Bipolar patients have elevated levels of oxidatively induced DNA base damages even during periods of stable mental state. The level of the base excision repair enzyme OGG1 that removes certain oxidized bases from DNA is also reduced compared to healthy individuals. Depressive disorder Major depressive disorder is associated with an increase in oxidative DNA damage. Increases in oxidative modifications of purines and pyrimidines in depressive patients may be due to impaired repair of oxidative DNA damages. Schizophrenia Postmortem studies of elderly patients with chronic schizophrenia showed that oxidative DNA damage is increased in the hippocampus region of the brain. The mean proportion of neurons with the oxidized DNA base 8-oxo-dG was 10-fold higher in patients with schizophrenia than in comparison individuals. Evidence indicating a role of oxidative DNA damage in schizophrenia has been reviewed by Raza et al. and Markkanen et al. RNA Oxidation RNAs in native milieu are exposed to various insults. Among these threats, oxidative stress is one of the major causes of damage to RNAs. The level of oxidative stress that a cell endures is reflected by the quantity of reactive oxygen species (ROS). ROS are generated from normal oxygen metabolism in cells and