AbstractThe structures of spherical RNA plant tiruses, containing 180 copies of one protein subunit, are closely related to those of the animal RNA picornaviruses, which are built of 60 copies of each of three larger capsid proteins. Other spherical RNA and DNA viruses utilize the same structural motif as building units of the viral capsid. It is therefore probable that many of the simple RNA viruses, whether found in animals or plants, have had a common genetic origin. The original gene may have coded for a protein that bound to specific molecules on the cell surface.