The genome incorporates at least 14 open reading frames (ORFs) that encode 28 proteins in three distinct classes: two large polyproteins P1a and P1ab that are cleaved into 16 non-structural proteins (nsp1–nsp16) during viral RNA synthesis; four structural proteins (S, E, M and N) that are necessary for viral entrance and gathering; and eight accessory proteins that are assumed to be dispensable for viral replication, but may facilitate viral assembly and take part in virulence and pathogenesis (Fig. 1 ).