XDH and synovial sarcoma: Elevated levels of enzymes that produce ROS and RNS, such as xanthine oxidase (XO), xanthine oxidoreductase (XOR), myeloperoxidase (MPO), NADPH oxidase 4 (NOX4), nitric oxide synthase 2 (NOS2), and nitric oxide synthase 3 (NOS3) were described in SS compared to healthy subjects [24,25,30,31].