u t p r r b e m Coronaviruses comprise a large and diverse family of enveloped, positive-stranded RNA viruses. The Coronaviridae exhibit broad host range, infecting many mammalian and avian species and causing upper respiratory, gastrointestinal, hepatic, and central nervous system diseases. In humans and fowl, coronaviruses primarily cause upper respiratory tract infections, while porcine and bovine coronaviruses establish enteric infections that result in severe economic loss. Coronaviruses of laboratory mice are, for historical reasons, designated as mouse hepatitis viruses (MHVs), but among these only a subset are strictly hepatotropic. Enteric strains are commonly found in rodent colonies and neurotropic strains are exploited to study central nervous system infection and demyelinating disease (Perlman et al., 2000). The extraordinary variations in host range and tissue tropism among coronaviruses are in large part attributable to variations in the spike glycoprotein. The S protein is a large, type I membrane glycoprotein that contains distinct functional domains near the amino (S1) and carboxy (S2) termini. These spikes function to define viral tropism by their receptor specificity and perhaps also by their membrane fusion activity during virus entry into cells. Recently their natural variation has attracted the attention of researchers interested in determinants of viral host range, virus entry, and virus–receptor interactions and their relationship to tropism. Evidence supporting a role for spike protein projections as agents of organ tropism and pathogenesis began with comparative studies of different naturally occurring MHV strains. In essence, nucleotide sequencing revealed that alterations in virus virulence were most closely associated with differences in the spike gene. These correlative findings were recently reinforced using the new technology of targeted RNA recombination, a