Distinguishing Epigenetic Marks Methylation of the cytosine base in eukaryotic DNA (5mC) is an important epigenetic mark involved in gene silencing and genome stability. Methylated cytosine can be enzymatically oxidized to 5-hydroxymethylcytosine (5hmC), which may function as a distinct epigenetic mark—possibly involved in pluripotency—and it may also be an intermediate in active DNA demethylation. To be able to detect 5hmC genome-wide and at single-base resolution, Booth et al. (p. 934 , published online 26 April) developed a 5hmC sequencing chemistry that selectively oxidizes 5hmC to 5-formylcytosine and then to uracil while leaving 5mC unchanged. Using this method, mouse embryonic stem cell genomic DNA was sequenced to reveal that 5hmC is found enriched at intragenic CpG islands and long interspersed nuclear element–1 retrotransposons.

Quantitative Sequencing of 5-Methylcytosine and 5-Hydroxymethylcytosine at Single-Base Resolution