IL-17F belongs to the IL-17 cytokine family which includes IL-17A, B, C, D, and E (also called IL-25). IL-17F shares the strongest homology to IL-17A. They share 50% amino acid sequence homology. The genes encoding IL-17 and IL-17F are localized in the same chromosomal region and are co-expressed by CD4+ and gamma delta T cells. Recently, an IL-17–IL-17F heterodimer was found to be expressed in Th17 cells together with IL-17 and IL-17F homodimers. Similar to IL-17, IL-17F utilizes IL-17RA and IL-17RC as its receptor and employs Act1 and TRAF6 as its signal transducers to induce the expression of pro-inflammatory cytokines and chemokines in many different cell types. IL-17F expression is upregulated in inflammatory bowel disease. A His161 to Arg161 (H161R) substitution in the third exon of the IL17F gene seems to be associated with asthma and chronic obstructive pulmonary disease (COPD) in Japanese subjects. In addition polymorphism of IL-17F seems to be associated to susceptibility to gastric cancer.