Interleukin 10 (IL-10), initially designated cytokine synthesis inhibitory factor (CSIF), was originally identified as a product of mouse T helper (Th) 2 clones that inhibited cytokine production by Th1 clones . The human ortholog of mouse IL-10 was subsequently cloned by cross-hybridization. Human and mouse IL-10 are 81% identical at the nucleotide and amino acid level. IL-10 is the prototypic member of the IL-10 cytokine family, including IL-10, IL-19, IL-20, IL-22 (IL-TIF), IL-24 and IL-26. A number of viruses, including Epstein-Barr virus (EBV) and human cytomegalovirus (HCMV), also encode viral members of the IL-10 family. Human IL-10 has cross-species actvity and is active on mouse cells. Mouse IL-10 is species-specific and does not act on human cells.