Methylation of DNA can occur non-enzymatically at the nitrogen-three of the cytosine base through spontaneous exposure to endogenous S-adenosyl methionine (SAM). The resulting 3-methylcytosine (3-mC) is mutagenic and must be repaired, which occurs in humans through the base excision repair (BER) or dealkylation via human homologues of the E. coli AlkB protein. 3-methylcytosine is present in human cell lines and increased levels of 3-mC impair proliferation.