Antineutrophil cytoplasmic antibody (ANCA)–associated vasculitis (AAV) is a severe systemic small-vessel disease characterized by the presence of autoantibodies such as autoantibodies against neutrophil proteins, leukocyte proteinase 3 (PR3-ANCA) and myeloperoxidase (MPO-ANCA) and categorized into granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA) and eosinophilic GPA (EGPA) based on clinical features [1]. The gene discussed is MPO; the disease is microscopic polyangiitis.