The potential effect of spontaneous post-mortem deamination of cytosine residues on the resolution of methylation patterns.

In fresh samples (left), methylated cytosine is resistant to bisulphite conversion and can be resolved after PCR and sequencing. Post-mortem deamination (DNA damage) is highlighted by arrowheads in the ancient sample (right). A proportion of unmethylated cytosine in ancient templates is sometimes converted to uracil and appears as thymine following PCR amplification [14]; this would not affect the resolution of methylation. However, post-mortem deamination of methylated cytosine to thymine would prevent the detection of methylation via bisulphite sequencing (circles).