Interleukin-1 Receptor Antagonist (IL-1RN) is a member of the IL-1 family. Endogenous IL-1RN is produced in numerous animal disease models as well as in human autoimmune and chronic inflammatory diseases. It binds to IL-1 receptors in competition with IL-1, but does not elicit intracellular response from this binding. Its role in counteracting the proinflammatory effects of IL-1 is being studied by numerous research groups. IL-4 and IL-13 have been shown to amplify the stimulatory effect of IL1-beta on the production of soluble and intracellular forms of IL-1RN. The regulated expression of IL-1RN in various cell types has been shown to be influenced by cytokines. In synovial fibroblasts, IL-1, TNF-alpha, or PDGF markedly enhances the synthesis of IL-1RN.