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). The gene discussed is PRTN3; the disease is granulomatosis with polyangiitis.