Interleukin-29, is a secreted protein which belongs to the IL-28 / IL-29 family, and is a type III interferon. It is also known as cytokine Zcyto21, Interferon lambda-1, IFN-lambda-1, IFNλ1, IL-29 and IFNL1, and is highly similar in amino acid sequence to the IL-28, the other type III interferon. IL28A, IL28B, and IL29, also named interferonλ2 (IFNλ2), IFNλ3, and IFNλ1, respectively, are classII cytokine receptor ligands that are distantly related to members of the IL 10 family (11-13% aa sequence identity) and the type I IFN family (15-19% aa sequence identity). Human IL29 cDNA encodes a 200 amino acid (aa) residue precursor protein with a putative 19 aa signal peptide and a 181 aa mature protein, which is a monomer in solution. It shares 67% and 69% aa sequence identity with human IL28A and IL28B, respectively. IL-29 plays an important role in host defenses against microbes and its gene is highly upregulated in cells infected with viruses and also may play a role in antiviral immunity. Interleukin-29 / IL-29 up-regulates MHC class I antigen expression. It is a Ligand for the heterodimeric class II cytokine receptor composed of IL10RB and IL28RA. The ligand/receptor complex seems to signal through the Jak-STAT pathway.