Methylation

thumb|An image of the 46 chromosomes making up the diploid genome of a human male (the mitochondrial chromosomes are not shown).

Methylation, in the chemical sciences, is the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These terms are commonly used in chemistry, biochemistry, soil science, and biology.

the covalent transfer of a methyl group to either N-6 of adenine or C-5 or N-4 of cytosine

'''S-Methylmethionine' (SMM) is a derivative of methionine with the chemical formula (CH3)2S+CH2CH2CH(NH3+)CO2−. This cation is a naturally-occurring intermediate in many biosynthetic pathways owing to the sulfonium functional group. It is biosynthesized from L-methionine and S-adenosylmethionine by the enzyme methionine S-methyltransferase. S''-methylmethionine is particularly abundant in plants, being more abundant than methionine.

class of enzymes

thumb|300x300px|SET7/9, a representative histone methyltransferase with SAM (blue) and peptide undergoing methylation (black). Rendered from PDB: 4J83. thumb|320px|The SN2-like methyl transfer reaction. Only the SAM cofactor and cytosine base are shown for simplicity.

named for the German chemist Wilhelm Eschweiler

ASH1L (also called huASH1, ASH1, ASH1L1, ASH1-like, or KMT2H) is a histone-lysine N-methyltransferase enzyme encoded by the ASH1L gene located at chromosomal band 1q22. ASH1L is the human homolog of Drosophila Ash1 (absent, small, or homeotic-like).