The definition of a virus as a DNA or RNA virus, based on its genetic material, is now on shaky ground thanks to the findings of Bresnahan and Shenk in the 30 June Science. Using a gene array, they find that particles of human cytomegalovirus (HCMV), a large DNA virus, contain four different mRNAs (Science 2000, 288:2373-2376). The mRNAs are derived from one immediate-early gene, two early genes and one late gene, but translation from at least one of the packaged mRNAs peaks before there is detectable translation from the most abundant immediate-early mRNA. The functions of the four mRNAs is unknown, but the protein product from one of the mRNAs is directed to the Golgi network. This co-translational sorting would not occur if the mRNA species was replaced in the virion by a protein.

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