Interleukin-23 (IL-23), a heterodimeric cytokine comprising IL-12p40 and IL-23p19 (Oppmann et al., 2000), is now well documented to be critical in the pathogenesis of a number of murine models of autoimmune and inflammatory conditions such as experimental autoimmune encephalomyelitis (EAE), collagen induced arthritis (CIA), and intestinal inflammation (Cua et al., 2003; Hue et al., 2006; Kullberg et al., 2006; Murphy et al., 2003; Uhlig et al., 2006b; Yen et al., 2006). This evidence concerns the gene IL23A and inflammation.