These clinical syndromes are frequently (but not always) characterized by ANCA reactive against one of two antigens, proteinase-3 (PR3) or myeloperoxidase (MPO), the former more commonly associated with GPA and the latter more frequently found in patients with MPA and EGPA (Finkielman et al. 2007). Here, MPO is linked to granulomatosis with polyangiitis.