IL-17A/F is part of the IL-17 cytokine family which consists of six structurally related proteins (IL-17A, B, C, D, E, and F). IL-17A is expressed primarily by Th17 cells, a subset of CD4 T cells. IL-17F is most closely related to IL-17A. The two molecules share 50% aminoacid sequence homology. Like IL-17A, IL-17F mRNA and protein have been detected in Th17 cells. IL-17F and IL-17A exist as a homodimers, adopting a cysteine knot motif formed through the interactions of four cysteines, one of which is responsible for the interchain bonding. IL-17F and IL-17A can form both homodimeric and heterodimeric proteins when expressed in a recombinant system and all forms of the recombinant proteins have in vitro functional activity. In addition, activated human CD4-T cells produce the homodimers of IL-17F, IL-17A, and the IL-17F/IL-17A heterodimer.