MPO and granulomatosis with polyangiitis: Antineutrophil cytoplasmic antibodies (ANCA) specific for proteinase 3 (PR3) and myeloperoxidase (MPO) are associated with necrotizing vasculitides, especially Wegener's granulomatosis (WG), microscopic polyangiitis (MPA) and idiopathic crescentic glomerulonephritis [1].