There are three types of interferons (IFN), alpha, beta and gamma. IFN-alpha is produced in the leukocytes infected with virus, while IFN-beta is from fibroblasts infected with virus. IFN-gamma is induced by the stimulation of sensitized lymphocytes with antigen or non-sensitized lymphocytes with mitogens. It is believed that IFN-alpha and beta originated from the same ancestral gene, whereas IFN-gamma did not. IFN has not only an antiviral activity, but also various kinds of biological activities including cell growth inhibition, immunosuppressive effects, enhancement of macrophage, natural killer (NK) cell, killer (K) cell and neutrophil functions, and cell differentiation-inducing activity. IFN also shows the antitumor activity resulting from the integration of the above-mentioned biological activities. IFN is also deeply involved in the pathogenesis of various diseases, e.g., collagen diseases such as SLE and rheumatoid arthritis, insulin-dependent diabetes mellitus, fulminant hepatitis, severe pancreatitis, nephritis, multiple sclerosis, allergic diseases, and atherosclerosis. At present, IFN is clinically used in therapy against virus infections such as hepatitis B and C, and for malignancies such as renal cell carcinoma, multiple myeloma, malignant melanoma, glioblastoma, skin cancers, malignant lymphoma and chronic myelogenous leukemia.