Figure 1.
Diagrammatic representation of CpG methylation (where cytosine (C) and guanine (G) are linked by a phosphate molecule). Approximately 60% of genes have CpG islands in a region called the promoter. This segment of DNA acts to control the activity, or expression, of that gene. An important property of CpG sites is that the cytosine can be methylated, by adding a methyl (CH3) molecule, to form methyl-CpG. Note that, because DNA is double stranded, with strands running in opposite directions, and cytosine bases on one strand pair with guanine on the other, CpG sites are methylated on both strands.