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