Nucleoproteins and nucleocapsids of negative-strand RNA viruses. 
A hallmark of negative-strand RNA viruses (NSVs) is that their genomes never exist as free RNA, but instead are always assembled with many copies of a single nucleoprotein (N) to form highly stable nucleocapsids.  Moreover, viral genomes are the only RNAs in infected cells that are assembled with N. The mechanism by which this specific association occurs, for both the segmented (s) and non-segmented (ns) viruses, has recently become clearer due to our expanding knowledge of N protein and nucleocapsid structures.