Interleukin 17F (IL-17F) is a 37 kD IL-17 family member. The IL-17 family consists of six members including IL-17 (also called IL-17A), IL-17B, IL-17C, IL-17D, IL-17E (also called IL-25), and IL-17F. IL-17F shares the strongest similarity to IL-17A and forms a homodimer or heterodimer with IL-17A. It is produced by Th17 cells, mast cells, basophils, and epithelial cells. IL-17F is an important regulator of inflammatory responses. It is involved in host defense against mucoepithelial infection by Staphylococcus aureus and Citrobacter rodentium. Over expression of IL-17F gene in the airway of mice is associated with airway neutrophilia, the induction of many cytokines, an increase in airway hyperreactivity, and mucus hyper-secretion. IL-17F is also involved in cancer immunity and autoimmune responses. IL-17F, like IL-17A, depends on IL-17R for its signaling in vitro and in vivo. P38 MAPK, ERK1/2, Act1 (NF-KB activator protein 1), and TRAF6 are involved in IL-17F signaling.