Interleukin-17 (IL-17 or IL-17A) is the founder member of the IL-17 cytokine family, which shares 50% homology with IL-17F. IL-17 cytokines are produced by T helper (TH)17 cells, gamma delta T cells, and other immune cells. IL-17A and IL-17F form homodimers and heterodimers as functional ligands. IL-17 cytokines bind to two receptors, IL-17RA and IL-17RC. IL-17RC forms a receptor heterodimer with IL-17RA. The receptor heterodimer is preferred by IL-17 ligands, thus, either IL-17RA knockout (KO) or IL-17RC KO completely abolishes IL-17 signaling. IL-17A and IL-17F are key proinflammatory cytokines involved in many inflammatory and autoimmune diseases.