<unk>  is a type of transferase enzyme which transfers a methyl group from a donor to an acceptor.
Often methylation occurs on nucleic bases in DNA or amino acids in protein structures.<unk>  uses a reactive methyl group bound to sulfur in S-adenosyl methionine (SAM) as the methyl donor.
DNA methylation is often utilized to silence and regulate genes without changing the original DNA sequence. This methylation occurs on cytosine residues. DNA methylation may be necessary for normal growth from embryonic stages in mammals. When mutant embryonic stem cells lacking the murine DNA<unk>  gene were introduced to a germline of mice they caused a recessive lethal phenotype.  Methylation may also be linked to cancer development as methylation of tumor suppressor genes promotes tumorgenesis and metastasis. 
Site-specific<unk> s have the same DNA target sequences as certain restriction enzymes. Methylation can also serve to protect DNA from enzymatic cleavage, since restriction enzymes are unable to bind and recognize externally modified sequences. This is especially useful in bacterial restriction modification systems which use restriction enzymes to cleave foreign DNA while keeping their own DNA protected by methylation.
Methylation of amino acids in the formation of proteins leads to more diversity of possible amino acids and therefore more diversity of function. The methylation reaction occurs on nitrogen atoms either on the N terminus or side-chain position of the protein and are usually irreversible.
Examples include:
DNA<unk> 
Histone<unk> 
5-Methyltetrahydrofolate-homocysteine<unk> 
Catechol-O-methyl transferase
5-hydroxyindole-O-methyltransferase
Methyltransferase
Ligase
Deoxyribonuclease
DNA glycosylase
Restriction enzyme
DNA polymerase
Oxidoreductase
Transaminase