Interleukin (IL)-23 is a recently identified member of the IL-12 family of heterodimeric cytokines that modulate subpopulations of T helper cells, and both IL-12 and IL-23 are attractive targets for therapy of autoimmune diseases. IL-23 is a binary complex of a four-helix bundle cytokine (p19) and a soluble class I cytokine receptor p40. IL-12 and IL-23 share p40 as an alpha-receptor subunit, yet show only 15% sequence homology between their four-helix cytokines p19 and p35, respectively, and signal through different combinations of shared receptors.