5-(Hydroxymethyl)cytosine-d2,13C is an isotope analogue of 5-(Hydroxymethyl)cytosine. 5-(Hydroxymethyl)cytosine is a DNA pyrimidine nitrogen base. It may regulate gene expression or prompt DNA demethylation. 5-Hydroxymethylcytosine may be especially important in the central nervous system, as it is found in very high levels there.