Interleukin-29, is a secreted protein which belongs to the IL-28 / IL-29 family, and is a cytokine with immunomodulatory activity. 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. 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.