Methylation is a form of alkylation , i.e the transfer of an alkyl group to another molecule . Methylation is specifically the addition or substition of a methyl group to a molecule. Methyl groups are alkyls made from methane and are carbon atoms attached to 3 hydrogen atoms -CH3.  It can be involved in the expression of genes , as well as protein function regulation and the metabolism of RNA .
- ↑ March's Advanced Organic Chemistry. Michael B. Smith, Jerry March - John Wiley & Sons (2007)