Human interleukin 20 (IL-20) is a protein belonging to the IL-10 family of cytokines. It exhibits approximately 28% amino acid (aa) identity with IL-10 and 76% aa identity with mouse IL-20. IL-20 is produced by activated keratinocytes and monocytes and transmits an intracellular signal through two distinct cell-surface receptor complexes on keratinocytes and other epithelial cells. A specific receptor for this cytokine is found to be expressed in skin and upregulated dramatically in psoriatic skin, suggesting a role for this protein in epidermal function and psoriasis. In addition, IL-20 regulates proliferation and differentiation of keratinocytes during inflammation, particularly inflammation associated with the skin. IL-20 also causes cell expansion of multipotential hematopoietic progenitor cells. The over-expression of both human and mouse forms of IL-20 results in keratinocyte hyperproliferation, abnormal epidermal differentiation, and neonatal lethality.