DNA methylation is one of the more widely studied and well-characterized of the main modes of epigenetic regulation. DNA methylation of cytosine residues is performed by one of a number of DNA methyltransferases which add a methyl group to a specific cytosine residue. These cytosine residues often reside in cytosine- and guanine-rich stretches of DNA called “CpG islands”. Sometimes DNA methylation is also referred to as CpG methylation. Generally, a DNA methylation-regulated gene whose promoter region shows a great degree of methylation will be effectively silenced. When the same gene’s promoter region is not methylated (i.e. the promoter is “hypomethylated”), the gene will likely not be silenced, and thus the gene will be expressed. Research has determined that the blocking of transcription in a methylated gene is not due to the methylation of DNA alone, but rather due to the irregular binding of a variety of proteins. In the presence of DNA methylation, proteins which normally bind DNA and enable transcription to proceed are unable to bind as well, or at all, which effectively limits or stops transcription. Research is
