Further supporting a central role of this pathway in chronic inflammatory disease susceptibility are the confirmed associations of the IL12B gene (encoding the p40 subunit of both IL-12 and IL-23 cytokines) with Crohn’s disease, ulcerative colitis and psoriasis; the JAK2 gene (a proximal kinase in the IL-23R pathway) in Crohn’s disease; and the STAT3 gene (immediately downstream of JAK2) in Crohn’s disease and ulcerative colitis [13], [14]. Here, JAK2 is linked to psoriasis.