DNA methylation

DNA methylation is an epigenetic modification in which methyl groups are added to cytosine bases, predominantly at CpG sites, by DNA methyltransferases. It regulates gene expression, X-inactivation, and genome stability without altering the underlying sequence. Methylation patterns shift predictably with age, forming the basis of epigenetic clocks such as Horvath's. Aberrant methylation contributes to cancer, immune dysfunction, and the broader epigenetic drift seen in ageing.