Modified versions of the four DNA bases — adenine, thymine, cytosine and guanine — can serve special purposes. Parasitic protozoa called trypanosomes have a modified version of thymine called J that has not been documented in other organisms.
In a study published in May, Anjana Rao of Harvard Medical School in Boston and her colleagues searched for enzymes similar to those responsible for making base J that might generate a similar base in mammals. They found TET1, which converts a methylated cytosine to hydroxymethylcytosine. This altered version accounts for 4–6% of all cytosines in the DNA of mouse embryonic stem cells. The researchers think that this enzyme may regulate gene activity through this modification.
Meanwhile, independently, Skirmantas Kriaucionis and Nathaniel Heintz at the Rockefeller University in New York identified the modified base in the mouse brain.