The IFN-lambda gene family is composed of three distinct genes: IFN-lambda1 (IL-29), IFN-lambda2 (IL-28A), and IFN-lambda3 (IL-28B). IFN-lambda proteins exhibit only weak homology to IL-10, but, like IL-10, they also use the IL-10R2 chain as a component of their receptor complex. The IFN-receptor complex consists of the unique ligand-binding chain IFN- R. The IFN-lambda-1 gene is found exclusively in primate genomes and is 81% identical to IFN-lambda-2/3. IFN-lambda-s, which are now collectively referred to as type III IFNs, exhibit several features in common with type I IFNs, such as antiviral, antiproliferative, and antitumor activities.