Interleukins 17F (IL-17F) and 17A (IL-17A) are closely related members of the IL-17 cytokine family, sharing about 50% amino acid identity. Studies in the mouse have identified Th17 cells as a distinct CD4+ T cell lineage that is defined by the production of IL-17F and IL-17A. IL-6 and transforming growth factor-β are required for the differentiation of naïve CD4+ T cells to Th17 cells, which are maintained in the presence of IL-23 and IL-1β. Conversely, IL-4 and interferon-γ can inhibit the development of Th17 cells. Th17 cells have been implicated in the pathology of mouse autoimmune disease models. Expression of IL-17F and IL-17A has been detected in activated Human peripheral blood lymphocytes. It has been shown that the cytokines are expressed in activated Human CD4+ T cells. Expression of IL-17F and IL-17A has also been observed in tissue samples from various autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, psoriasis, inflammatory bowel disease, and asthma. The crystal structure of IL-17F has been solved and shows that the protein forms a disulfidelinked dimeric glycoprotein. IL-17A is also a disulfide-linked homodimeric glycoprotein. The IL- 17F homodimer includes a classical cysteine knot motif, which is found in the TGF-β, bone morphogenetic protein, and nerve growth factor superfamilies. One difference in the cysteine knot motif of IL-17F compared with the other known cysteine knot protein families is that it only utilizes four cysteines instead of the classical six cysteines to form the knot. IL-17F and IL-17A have been shown to form biologically active IL-17F/IL-17A heterodimers, in addition to the IL- 17F and IL-17A homodimers.