DNA base pairs can be damaged by reactive oxygen species. Cells have systems in place to excise damaged bases which results in the production and subsequent excretion of the damaged portion of DNA.
One such system, OGG1 acts to remove oxidized guanine in the form of 8-hydroxydeoxyguanosine (8OHDG). Other byproducts of oxidative stress, specifically malondialdehyde due to lipid peroxidation can react with oxidized guanine to form MDA-8OHDG, another biomarker of oxidative mediated damage to DNA.