IFNG and severe acute respiratory syndrome: To understand the role of type I (IFNα/β) and type II IFN (IFNγ) in the response to SARS-CoV infection we infected IFN receptor −/− mice with either a late phase SARS-CoV (Urbani) virus or a recombinant isogenic mouse-adapted virus (rMA15) that contained six virulence modifying mutations [30], anticipating that the mouse-adapted virus may display more prominent pathogenicity than the Urbani virus.