DNA methylation is an epigenetic process, it doesn't change the DNA sequence itself, but adds chemical "tags" (methyl groups) to the DNA that tell genes to be more or less active. Think of it like highlighter marks on a textbook, the words don't change, but the highlighted passages get read more often.
In MCHS context: The episignature paper identified a unique methylation pattern specific to MEF2C deficiency. This methylation pattern acts as a biomarker — a signature you can detect in a blood test to confirm or support an MEF2C diagnosis. This is potentially transformative for cases where genetic testing is ambiguous.