Interleukin 23 (IL-23) is a heterodimeric cytokine that is related to IL-12. IL-23 is a heterodimeric cytokine composed of an IL-12p40 subunit that is shared with IL-12 and the IL-23p19 subunit. A functional receptor for IL-23 (the IL-23 receptor) has been identified and is composed of IL-12Rβ1 and IL-23R. Human IL-23 has biological properties which are less distinct from human IL-12; human IL-23 induces proliferation of memory T cells and induces moderate levels of IFN-γ production by naïve and memory T cells, as compared to IL-12. IL-23 effects on IL-17-producing T cells may also enhance the development of several models of autoimmune disease including experimental allergic encephalomyelitis (EAE), collagen-induced arthritis (CIA), colitis, and diabetes . IL-23 may also play a role in increased tumor growth associated with chronic inflammation . In humans, IL-23 has been found upregulated in several pathologies with dysregulated immune function including psoriasis, Crohn’s disease, and multiple sclerosis .