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Coronaviruses were recognized as a group in 1968 primarily on the basis oftheir characteristic morphology as seen in the electron microscope (Tyrrell et al. 1968). Since that time our knowledge of the structure and replication of these viruses has increased steadily and has been periodically reviewed (Mdntosh 1974; Tyrrell et al. 1978; Robb and Bond 1979a). The basis for this review, which concentrates on the molecular biology of coronaviruses, is principally the new data which has become available in the last 2 years. The pathogenicity of these viruses, which are associated with many diseases of clinical importance in animals and humans, is the subject of the accompanying article. Coronaviruses infect a wide variety of animal species ranging from fowl to humans. The group comprises 11 recognized viruses, and five more that are tentatively included

The family Coronavirtdae comprises a monogeneric group of 11 viruses which infect vertebrates. The main characteristics of the member viruses are: (i) Morphological: Enveloped pleomorphic particles typically 100 nm in diameter (range 60–220 nm), bearing about 20 nm long club-shaped surface projections, (ii) Structural: A single-stranded infectious molecule of genomic RNA of about (5–7) × 10⁶ molecular weight. A phosphorylated nucleocapsid protein [mol.wt. (50–60) × 10³] complexed with the genome as a helical ribonucleoprotein; a surface (peplomer) protein, associated with one or two glycosylated polypeptides [mol.wt. (90–180) × 10³]; a transmembrane (matrix) protein, associated with one polypeptide which may be glycosylated to different degrees [mol.wt. (20–35) × 10³]. (iii) Replicative: Production in infected cells of multiple 3’ coterminal sub genomic mRNAs extending for different lengths in the 5’ direction. Virions bud intracytoplasmically. (iv) Antigenic: 3 major antigens, each corresponding to one class of virion protein, (v) Biological: Predominantly restricted to infection of natural vertebrate hosts by horizontal transmission via the fecal/oral route. Responsible mainly for respiratory and gastrointestinal disorders.

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