Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is characterized by the destruction of small and medium-sized arterial vessels with few or no immune deposits, in the presence of circulating autoantibodies toward the cytoplasmic region of the neutrophil (ANCA) predominantly against proteinase-3 (PR3) and myeloperoxidase (MPO). The gene discussed is MPO; the disease is anti-neutrophil cytoplasmic antibody-associated vasculitis.