The immune suppressive pharmacodynamic effect of thiopurines in IBD dosages (2–2.5 mg/kg bodyweight for AZA, 1–1.5 mg/kg bodyweight for MP and 0.2–0.3 mg/kg bodyweight for TG) is primarily based on inhibition of the small GTPase Ras-related C3 botulinum toxin substrate 1 (Rac1) causing apoptosis of activated T lymphocytes (Fig. 2)18. The gene discussed is RAC1; the disease is inflammatory bowel disease.