Microscopic polyangiitis (MPA) is a type of AAV marked by renal involvement, which often leads to rapidly progressive glomerulonephritis and the presence of myeloperoxidase-ANCA (MPO-ANCA; although proteinase 3 (PR3)-ANCA has also been reported). The gene discussed is MPO; the disease is anti-neutrophil cytoplasmic antibody-associated vasculitis.