Efficacy and Safety of GLPG0634 in Subjects With Active Crohn's Disease

* 180 patients suffering from active Crohn's disease with evidence of mucosal ulceration will be evaluated for improvement of disease activity (efficacy) when taking GLPG0634 or matching placebo once daily for 20 weeks in addition to their stable background treatment. * During the course of the study, patients will also be examined for any side effects that may occur (safety and tolerability), and the amount of GLPG0634 present in the blood (Pharmacokinetics) as well as the effects of GLPG0634 on disease- and mechanism of action-related parameters in the blood and stool (Pharmacodynamics) will be determined. Also, the effects GLPG0634 administration on subjects' quality of life will be evaluated.