IL-17A was initially identified from a subtracted cDNA library between closely related murine lymphoid cells and called CTLA-8, It also shares 58% homology with an open reading frame of the T-lymphotropic Herpesvirus Samirii virus (viral IL-17). IL-17A belongs to a family of cytokines, which has six members; designated IL-17A-F. IL-17 is expressed by a unique lineage of CD4+ T cells (Th17) that develop in response to IL-23, in particular under conditions in which Th1 and Th2 development are suppressed. IL-17A shares the greatest homology (55%) with IL-17F. Both IL-17A and IL-17F are produced by Th17 cells. IL-17A and IL-17F can either exist as IL-17A homodimers and IL-17F homodimers or as IL-17A-IL-17F heterodimers. IL-17 is a key mediator of autoimmune disorders, including rheumatoid arthritis, psoriasis, inflammatory bowel disease, and asthma, and plays a role in host defense.