The five IL-17 receptors (IL-17RA IL-17RE) are not homologous to any known receptors, and exhibit considerable sequence divergence between one another. They all contain extracellular domains composed of fibronectin type-III (FnIII) domains, and cytoplasmic SEF/IL-17R (SEFIR) domains that show loose homology to Toll/IL-1R (TLR) domains. The IL-17RA ectodomain is composed of two unusual FnIII domain modules joined by an 18-amino acid linker, members of this superfamily share the structure of the second FnIII domain (D2). The two linked fibronectin-type domains bind two molecules of IL-17 at one of its receptor-binding interfaces. This allows the other interface to bind to another receptor, thus allowing the IL-17 family of homodimeric cytokines to coordinate two different receptors PMID:23695682,PMID:19838198.