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-(gamma)) binds to a distinct receptor. This distinct receptor, IFN-(alpha)R, is only weakly responsive to type I interferons, in contrast to IFN-(alpha)/(beta)R, which binds to and responds effectively to IFN-(beta) and to several of the IFN-(alpha) subtypes. IFN-(alpha)/(beta)R is expressed as two alternatively spliced transcripts, designated IFN-(alpha)/(beta)R(alpha) (IFN-(alpha)/(beta)R1) and IFN-(alpha)/(beta)R(beta) (IFN- (alpha)/(beta)R2), both of which are involved in signal transduction and ligand binding.