The importance of DNA methylation has been intensely studied over many years. Recently the dichotomy of cytosine:methyl cytosine was shattered by the discovery of 5-hydroxymethylcytosine (5-hmC) in mammalian genomic DNA. Produced by oxidation of 5-methylcytosine by the TET family of enzymes, this "new” epigenetic modification is a major focus of epigenetics research with potential roles in development and differentiation through transcriptional regulation and demethylation.