Methylation of DNA plays a major role in many biological processes throughout phylogeny. Generally, methylated DNA is associated with inactive gene transcription and compact chromatin structure. In mammalian DNA, typically cytosine residues within the 5′-CpG-3′ dinucleotide have the potential to be methylated. By contrast, methylation of cytosine residues within the context of other nucleotide sequences such as 5′-CCTGG-3′ is infrequent (1 ).
