3-methylcytosine arises by erroneous methylation of DNA through spontaneous exposure to endogenous S-adenosyl methionine (SAM). Since the N3-position in cytosine is protected in double stranded DNA, this phenomenon mainly occurs in single stranded DNA. The resulting 3-mC is toxic and mutagenic and increased levels of 3-mC seem to impair cell proliferation. Therefore, it has to be repaired by the cell. This is established by oxidative demethylation, which is catalyzed by the AlkB protein.