IFNs are a large family of proteins having antiviral, antiproliferative, and immunomodulatory effects, and are divided into two major classes, type I and type II, on the basis of differences in receptor binding and nucleotide sequence. Type I IFNs consist of IFN α, β, τ, and ω and bind to the type I IFN receptor, whereas IFN-γ is the only type II IFN and is specific for the type II IFN receptor. Human IFN-ω, was identified by three independent groups in 1985 and is structurally related to IFN-α and -β. Both human IFN-ω and IFN-α are produced by virally induced leukocytes and have similar antiviral activities on human cell lines, and a sizeable proportion (at least 1%) of the total antiviral activity of leukocyte IFN is contributed by IFN-ωl. In addition, it was reported that IFN-ω could inhibit the growth of human tumors in vivo.