AAV can be further subdivided in three diagnostic entities, granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA) and eosinophilic granulomatosis with polyangiitis (EGPA) with the presence of ANCA autoantibodies targeting the myeloperoxidase (MPO) or the neutrophil cytoplasmic antigens proteinase 3 (PR3). The gene discussed is MPO; the disease is granulomatosis with polyangiitis.