MPO and granulomatosis with polyangiitis: ANCA and, more specifically, the enzymes myeloperoxidase (MPO) and proteinase 3 (PR3) are found in autoimmune vasculitis, which is known as ANCA-associated vasculitis and includes granulomatosis with polyangiitis, microscopic polyangiitis, and eosinophilic granulomatosis [6].