FIG. 1.

SARS-CoV and MHV genome structure. The genome structure of coronaviruses is very conserved among all known coronaviruses. In each coronavirus, the N-terminal two-thirds of the genome encodes the nonstructural proteins, also called the replicase proteins (orange box). The C-terminal one-third of the genome encodes the structural (red boxes) and accessory (gray boxes) ORFs. The structural ORFs encode the spike, envelope (E), membrane (M), and nucleocapsid (N) proteins. Each coronavirus has similar structural ORFs in their genomes. The accessory ORFs, in gray, are unique to each coronavirus. There is no sequence or structural similarities between the MHV and SARS-CoV accessory proteins.