The genes encoding type I interferons (IFNs), which include 14 IFN-a genes, one IFN-? gene, one IFN-w (also known as IFN-a II1) gene, and a number of IFN-w pseudogenes, are clustered on human chromosome 9. Interferon-a and -? 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-a, IFN-? or IFN-g, 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-aR) interacts with IFN-a, IFN-? and IFN-w, and seems to be a multisubunit receptor.