: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. Brierleespecially: DNA METHYLATIONAs cells begin to specialize into adult tissues, methylation seems to inactivate genes that are no longer needed.— John Travis
