The genes encoding type I interferons (IFNs), which include 14 IFN-alphagenes (two of which are IFN-alpha1 and IFN-alpha13), one IFN-∫ gene, one IFN-w (also known as IFN-alpha II1) gene and a number of IFN-w pseudogenes, are clustered on human chromosome 9. IFN-alpha and -beta are cytokines that are widely known to induce potent antiviral activity. They exert a variety of other biological effects, including antitumor and immunomodulatory activities and are increasingly used clinically to treat a range of malignancies, myelodysplasias and autoimmune diseases. IFN-w is antigenically different from human IFN-alpha, IFN-beta or IFN-gamma, but is a component of natural mixtures of IFN species produced by virus-induced leukocytes or Burkitt's lymphoma cells. The type I interferon receptor (IFN-alphaR) interacts with IFN-alpha, IFN-beta and IFN-w, and seems to be a multisubunit receptor.