Oxoguanine glycosylase (OGG1) catalyses the first step in base excision repair of the oxidatively damaged base 8-OHdG. OGG1 finds 8-OHdG by sliding along the linear DNA at 1,000 base pairs of DNA in 0.1 seconds. OGG1 very rapidly finds 8-OHdG. OGG1 proteins bind to oxidatively damaged DNA with a half maximum time of about 6 seconds. When OGG1 finds 8-OHdG it changes conformation and complexes with 8-OHdG in its binding pocket. OGG1 does not immediately act to remove the 8-OHdG. Half maximum removal of 8-OHdG takes about 30 minutes in HeLa cells in vitro, or about 11 minutes in the livers of irradiated mice. DNA oxidation by reactive oxygen species preferentially occurs at a guanine in a methylated CpG site, because of a lowered ionization potential of guanine bases adjacent to 5-methylcytosine. TET1 binds (is recruited to) the OGG1 bound to 8-OHdG (see figure). This likely allows TET1 to demethylate an adjacent methylated cytosine. When human mammary epithelial cells (MCF-10A) were treated with HO, 8-OHdG increased in DNA by 3.5-fold and this caused about 80% demethylation of the 5-methylcytosines in the MCF-10A genome.