IL-20 is a member of the IL-10 cytokine family that is expressed by monocytes, dendritic cells, and keratinocytes. IL-20 activates STAT3 by activating either of two heterodimeric receptor complexes. One of these consists of IL-20Rβ and IL-20Rα that can be activated by IL-19 and IL-24 as well as IL-20. The other, a complex of IL-20Rβ and IL-22Rα that is activated by only IL-20 and IL-24. IL-20 ligand binding induces the production of proinflammatory cytokines such as IL-1β, IL-6, and TNFα in monocytes and has been implicated in inflammatory conditions such as atherosclerosis, rheumatoid arthritis, and psoriasis. IL-20 also induces the proliferation of hematopoietic progenitor cells and keratinocytes.