The antineutrophil cytoplasmic antibody (ANCA)–associated vasculitides, which include ANCA–associated vasculitis (AAV), are a group of systemic autoimmune diseases characterized by the presence of autoantibodies directed against proteinase 3 (PR3) or myeloperoxidase (MPO) (1–3) in the serum of most patients. The gene discussed is MPO; the disease is anti-neutrophil cytoplasmic antibody-associated vasculitis.