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Correction of small base changes, e.g. oxidation, alkylation.

The first step of BER is removal of the damaged base by a glycosylase, leaving an apurinic/apyrimidinic or AP-site. The glycosylases are more or less lesion-specific. For example, there is a human enzyme, hOGG1, that removes 8-oxoguanine. The glycosylases often have an associated AP lyase activity which completes the DNA breakage. This may be very slow. An AP endonuclease speeds up the process. After conversion of the AP site to a break, the gap is filled by a DNA polymerase, inserting the correct nucleotide, and the original DNA sequence and structure is restored with the final ligation step.