IDO1 and rheumatoid arthritis: In this article, we discuss the detailed characteristics of these major enzymes and their derivatives including phospholipase A2 (PLA2), cyclooxygenase (COX) and prostaglandins (PGs), lipoxygenase (LOX), matrix metalloproteases (MMPs)/tissue inhibitors of metalloproteases (TIMPs), nitric oxide synthase (NOS) and nitric oxide (NO), as well as indoleamine 2, 3-dioxygenase (IDO), and demonstrate the targeting of these spots by natural herbal and synthetic products resulting in the prevention of RA (Table 1).