The IL-29 gene is found on chromosome 19 in humans. It is also known as IFN-λ1 and is highly similar in amino acid sequence to the IL-28, the other type III interferon. IL-29 plays an important role in host defenses against microbes and its gene is highly upregulated in cells infected with viruses. IL-29 has significant antiviral activity and immunoregulatory properties and appears to inhibit T helper-2 (Th2) responses regarding inhibition of IL-13 production, compared with IL-4 or IL-5.  IL-29 inhibits virus replication by inducing a cellular antiviral response similar to that activated by IFN-α/β. However, because it binds to a unique receptor, this cytokine may function cooperatively with IFN-α/β or IFN-γ during natural infections to inhibit virus replication, and might also be useful therapeutically in combination with other cytokines to treat chronic viral infections such as hepatitis C (HCV).The antiviral activities of IL-29 include the upregulation of MHC Class I expression on the cell surface and the expression of PKR. The ligand/receptor complex seems to signal through the Jak-STAT pathway. IL-29 is not present in the mouse genome.