Fig. 2.

DNA methylation and gene expression. A simplified schematic of DNA methylation and its effect on gene expression. DNA methylation occurs by the covalent addition of a methyl group to the 5′ carbon of a cytosine nucleotide. Methylated CpG sites (filled lollipop) are associated with gene silencing, whereas unmethylated sites (unfilled lollipop) are associated with transcriptional activity. In the case of hydroxymethylation, at the 5′ carbon position, the hydrogen molecule is replaced by a hydroxymethyl group