Coronaviruses form enveloped and spherical particles of 100–160 nm in diameter. They contain a positive-sense, single-stranded RNA (ssRNA) genome of 27–32 kb in size. The 5'-terminal two-thirds of the genome encodes a polyprotein, pp1ab, which is further cleaved into 16 non-structural proteins that are involved in genome transcription and replication. The 3' terminus encodes structural proteins, including envelope glycoproteins spike (S), envelope (E), membrane (M) and nucleocapsid (N). In addition to the genes encoding structural proteins, there are accessory genes that are species-specific and dispensable for virus replication. Here, we compare prototypical and representative strains of four coronavirus genera: feline infectious peritonitis virus (FIPV), Rhinolophus bat coronavirus HKU2, severe acute respiratory syndrome coronavirus (SARS-CoV) strains GD02 and SZ3 from humans infected during the early phase of the SARS epidemic and from civets, respectively, SARS-CoV strain hTor02 from humans infected during the middle and late phases of the SARS epidemic, bat SARS-related coronavirus (SARSr-CoV) strain WIV1, Middle East respiratory syndrome coronavirus (MERS-CoV), mouse hepatitis virus (MHV), infectious bronchitis virus (IBV) and bulbul coronavirus HKU11.