The purpose of study is to test the effects of an experimental medication GED-0301 (mongersen) in patients who have active Crohn's disease. The study will test GED-0301 compared to placebo for 52 weeks. The study treatment is blinded which means that patients and the study doctor will not know which treatment has been assigned. Patients in this study will be allowed treatment with stable doses of oral aminosalicylates, oral corticosteroids, immunosupressants and antibiotics for the treatment of Crohn's disease.