The type I interferons, IFN-alpha and IFN-beta, are a group of structurally and functionally related proteins that are induced by either viruses or double-stranded RNA and are defined by their ability to confer an antiviral state in cells. IFN-alpha and IFN-beta appear to compete with one another for binding to a common cell surface receptor, while immune IFN (IFN-y) binds to a distinct receptor. This distinct receptor, IFN-alphaR, is only weakly responsive to type I interferons, in contrast to IFN-alpha/betaR, which binds to and responds effectively to IFN-beta and to several of the IFN-alpha subtypes. IFN-alpha/betaR is expressed as two alternatively spliced transcripts, designated IFN-alpha/betaRalpha (IFN-alpha/betaR1) and IFN-alpha/betaRbeta (IFN- alpha/betaR2), both of which are involved in signal transduction and ligand binding.