Gene expression can be regulated without changing the DNA sequence through what is called an 'epigenetic mechanism' that frequently involves 'methylation'. The attachment of a methyl group (-CH3) to one of the DNA building blocks (cytosine) stops the gene being read. These methyl groups come from choline, betaine, folate and vitamin B12.
: the introduction of a methyl radical into a substance The methylation of metals (that is, the substitution of a metal atom for the hydrogen atom of the hydroxyl group of a methyl alcohol molecule) can result in the metal's becoming volatile.
— Corale L. Brierle
especially: DNA METHYLATION
As cells begin to specialize into adult tissues, methylation seems to inactivate genes that are no longer needed.
— John Travis