Science doi:10.1126/science.1170116 (2009); Science doi:10.1126/science.1169786 (2009)

Although four DNA bases — adenine, thymine, cytosine and guanine — make up much of the genome, modified bases can serve special purposes. Trypanosomes, parasitic protozoa, contain an additional base called J that is a hypermodified version of thymine that has not been documented in other organisms.

Anjana Rao of Harvard Medical School and her colleagues searched for enzymes similar to those responsible for making base J that might make a similar base in mammals. They found TET1, which makes a modification to cytosine to create hydroxymethylcytosine. This accounts for 4–6% of all cytosines in the DNA of mouse embryonic stem cells.

Meanwhile, independently, Skirmantas Kriaucionis and Nathaniel Heintz at the Rockefeller University in New York have identified the modified base in the mouse brain.