MPO and neoplasm: The inflammatory cells in the tumor microenvironment lead to a massive production of ROS by activating the oxidant-generating enzymes such as inducible nitric oxide synthase (iNOS), myeloperoxidase (MPO), NADPH oxidase, and xanthine oxidase (XO), and by up-regulating cyclooxygenase 2 (COX2) and lipoxygenase (LOX) to remove and disrupt the biological, chemical, and physical factors.