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. An example of this is the tri-methylation of lysine 36 on the H3 protein (of a histone), which is involved in the response of plants to necrotrophic fungal attack.