DNA methylation occurs mainly at the 5’-position of cytosine rings (5-methylcytosine) and occurs almost exclusively in CpG islands. Another epigenetic modification in DNA has been recently identified that involves hydroxymethylation of this same base (5-hydroxymethylcytosine), which potentially offers another level of transcriptional control. 5-(Hydroxymethyl)-2’-deoxycytidine is a modified pyrimidine that is capable of producing interstrand cross-links in double-stranded DNA and has been used to quantify DNA hydroxymethylation levels in biological samples.
