MPO (myeloperoxidase) is a peroxidase enzyme secreted by activated leukocytes that plays a pathogenic role in cardiovascular disease, mainly by initiating endothelial dysfunction. Myeloperoxidase (MPO) is an important enzyme, which is one of the components of the antibacterial system in neutrophils and monocytes. MPO participates in the inflammatory response in multiple locations in the body, including the mammary glands. Myeloperoxidase (MPO), a specific polymorphonuclear leukocyte enzyme, has been used previously to quantify the number of neutrophils in tissue. MPO activity was found to be linearly related to the number of neutrophil cells. The MPO system plays an important role in the control of infections and the deletion of malignant cells. Nevertheless, alternations in the MPO system can lead to DNA damage and carcinogenesis. Polymorphisms in the MPO gene have been associated with an increased expression of MPO and a higher risk for the development of cancer. Myeloperoxidase (MPO) is one of the major target antigens of antineutrophil cytoplasmic autoantibodies (ANCA) found in patients with small-vessel vasculitis and Pauci-immune necrotizing glomerulonephritis. Myeloperoxidase-anti-neutrophil cytoplasmic antibody (MPO-ANCA) is an autoantibody that is frequently found in patients with vasculitides.