Methylation is a biochemical process in which a methyl group (CH3) is added to a molecule, typically DNA or RNA. This addition can have various effects on the function and activity of the target molecule, depending on where the methyl group is attached. In some cases, it can lead to changes in gene expression, while in other instances it may help protect genetic material from damage. Methylation plays an important role in many biological processes, including normal development, cellular differentiation and regulation of gene activity.