PRTN3 and microscopic polyangiitis: Microscopic polyangiitis is associated with proteinase 3 (PR3)-ANCA in 26% of cases and with myeloperoxidase (MPO)-ANCA in 58% of cases (5), while GPA is characterized by PR3-ANCA in 66% of patients and MPO-ANCA in 24% of patients (6).