<p>Arterivirus are ssRNA positive-strand viruses with no DNA stage in their replication cycle. This family contains the viral nucleocapsid protein, which encapsidates the viral ssRNA.</p><p> <taxon tax_id="28344">Porcine reproductive and respiratory syndrome virus</taxon> (PRRSV) is the causative agent of both severe and persistent respiratory disease and reproductive failure in pigs worldwide. The PRRSV virion contains a core made of the 123 amino acid nucleocapsid (N or VP1) protein, a product of the ORF7 gene. The crystal structure of the capsid-forming domain of the nucleocapsid protein has been determined to 2.6 A resolution. The protein exists as a tight dimer forming a four-stranded beta sheet floor superposed by two long alpha helices and flanked by two N- and two C-terminal alpha helices. The structure represents a new class of viral capsid-forming domains, distinctly different from those of other known enveloped viruses, but reminiscent of the coat protein of bacteriophage MS2 [<cite idref="PUB00028004"/>].</p>