In a paper published online in advance in Science this week, researchers at the University of Virginia School of Medicine and the University of North Carolina at Chapel Hill, report on their identification of tens of thousands of "short extrachromosomal circular DNAs — microDNA — in mouse tissues as well as mouse and human cell lines." The team says these 200- to 400-base pair long microDNAs are "derived from unique nonrepetitive sequence, and are enriched in the 5' untransla