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Front cover illustration

Expression of the polycistronic genome of nidoviruses (corona-, toro-, arteri- and roniviruses) is achieved and regulated by the synthesis of a nested set of mRNAs, consisting of the viral genome and two to nine subgenomic transcripts. Genome translation yields the large replicase polyproteins that - after autoproteolytic processing - establish the viral replication complex and drive viral RNA synthesis. The subgenomic mRNAs mainly serve to express the structural proteins. These immunofluorescence microscopy images of Vero-E6 cells infected with SARS-coronavirus illustrate the expression of four important viral genes and the different subcellular localization of their products. From top to bottom: non-structural protein 3, one of the replicase subunits translated from genomic RNA, revealing the localization of the membrane-bound viral replication complex; spike protein S, produced from subgenomic mRNA2 and localized to different compartments of the exocytic pathway; membrane protein M, the translation product of subgenomic mRNA5, which accumulates in the Golgi complex, in particular early in infection; and nucleocapsid protein N, a largely cytoplasmic protein that is produced from subgenomic mRNA9. Images courtesy Yvonne van der Meer, Jessika Zevenhoven-Dobbe and Eric Snijder, Department of Medical Microbiology, Leiden University Medical Center, the Netherlands. See the review by Pasternak et al. in this issue, pp. 1403-1421.