The 5′-proximal two-thirds of the coronavirus genome encodes the replicase gene, which contains two open reading frames, ORF1a and ORF1b. Translation of ORF1a yields polyprotein 1a (pp1a), and  -1 ribosomal frameshifting allows translation of ORF1b to yield pp1ab. Together, these polyproteins are co- and posttranslationally processed into 16 nonstructural proteins (nsps), most of them driving viral genome replication and subgenomic mRNA (sgmRNA) synthesis. The 3′ third of the genome encodes the structural and accessory proteins, which vary in number among the different coronaviruses.