JAK2 and rheumatoid arthritis: Tofacitinib citrate selectively inhibits JAK1 and JAK3, and to a lesser extent than JAK2, and is approved by the European Medicines Agency and U.S. Food and Drug Administration for various autoimmune diseases, including rheumatoid arthritis, psoriatic arthritis, ulcerative colitis, and polyarticular juvenile idiopathic arthritis.