The virion RNA is infectious and serves as both the genome and viral messenger RNA. The genome contains three overlapping open reading frames (ORF1a, ORF1b, and ORF2). The nonstructural proteins are translated from the genomic RNA as two large polyproteins, nsP1a and nsP1a/1b, through a translational ribosomal frameshifting mechanism. ORF1a and ORF1b encode the viral protease and polymerase respectively (and probably other proteins involved in genome replication). ORF2 is expressed from a subgenomic RNA and encodes the VP90 capsid precursor protein.