DNA methylation is an epigenetic mechanism that involves the addition of methyl groups to a DNA molecule. Of the four eukaryotic DNA bases, cytosine and adenine can be methylated.

A key characteristic of DNA methylation is that it does not change the sequence of the DNA molecule. Rather the addition of methyl groups can alter the activity of a DNA molecule, repress gene transcription and is associated with a variety of developmental processes (e.g. genomic imprinting, X-chromosome inactivation, aging and carcinogensis).