DNA methyltransferases (DNMTs) are a family of enzymes that catalyze the transfer of a methyl group from the methyl donor, usually S-adenosyl-L-methionine (SAM) for all known DNMTs, to the 5-position of cytosine residues in DNA. In mammals, four members of the DNMT gene family have been identified and designated, DNMT1, DNMT3A, DNMT3B and DNMT3L respectively, in which DNMT1 encodes the maintenance methyltransferase, DNMT3A/3B encode the de novo methyltransferases, and DNMT3L encodes an enzyme without any inherent enzymatic activity. Mammalian DNMTs are involved in transcriptional silencing of several sequence classes and play an important role in maintaining genome stability and genomic integrity.