MPO and anti-neutrophil cytoplasmic antibody-associated vasculitis: Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a systemic autoimmune disorder involving a pauci-necrotizing small-sized vasculitis, in which ANCA targeting myeloperoxidase (MPO) and proteinase 3 (PR3) participate in the pathogenesis of the disease.