Redefining virology.

Viruses have always been classified according to whether their genome is composed of DNA or RNA. That may be set to change with the discovery that human cytomegalovirus has both a DNA genome and four mRNA transcripts that are produced before the DNA genome is transcribed after infection of the host cell (Bresnahan and Shenk). As Roizman points out in a lively Perspective, finding out what the proteins encoded by these four mRNAs do, and whether other DNA viruses show this sneaky partnering of DNA and RNA, will keep virologists busy for many years to come.