<h4>Background</h4>Microscopic polyangiitis (MPA), a severe antineutrophil cytoplasmic antibody associated vasculitis (ANCA-associated vasculitis, AAV), demonstrates strong clinical association with myeloperoxidase/perinuclear anti-neutrophilic cytoplasmic antibodies (MPO/P-ANCA). The gene discussed is MPO; the disease is microscopic polyangiitis.