Interleukin 29 (IL-29), also known as interferon λ1 (IFN-λ1), is a member of the helical cytokine family and is a type III interferon. Its amino acid sequence is highly similar to that of another type III interferon, IL-28.The IL-29 gene is found on human chromosome 19 but is absent from the mouse genome.IL-29 plays an important role in host defense against microorganisms, and its gene is highly up-regulated in virus-infected cells.IL-29 possesses significant antiviral activity and immunomodulatory properties, and can be activated by induction of IFN-α/β-like activation of cellular antiviral responses to inhibit viral replication. However, IL-29 is bound to a unique receptor, and thus this cytokine may act synergistically with IFN-α/β or IFN-γ to inhibit viral replication during natural infections, or in combination with other cytokines for the treatment of chronic viral infections such as hepatitis C (HCV).The antiviral activity of IL-29 includes up-regulation of class I MHC expression on cell surface and upregulation of PKR expression. The ligand/receptor complex appears to signal through the Jak-STAT pathway.