The purpose of this study is to measure the mucosal immunological effects and peripheral blood effects of the oral IL-12/23 inhibitor, STA-5326 mesylate, in subjects with active Crohn's disease (CD). Crohn's disease is a chronic, relapsing inflammatory condition of the small and large bowel associated with excessive Th1 cytokine production. Since IL-12 is a key cytokine driving the Th1 response, novel therapies targeting IL-12 have been successful in treating CD in animal models and in humans. Data from humans and from animal models further suggest that both IL-12 and IL-23 may have a role in this active gut inflammation. STA-5326 mesylate is an oral drug that suppresses IL-12p40 production (a subunit shared by IL-12 and IL-23) in vitro and has apparent safety and clinical activity in a recent open-label trial of treatment for CD.