PR3-ANCA are an established marker for the diagnosis of small vessel vasculitis including granulomatosis with polyangiitis (GPA) (formerly Wegener's granulomatosus); MPO is the most frequently identified antigen in pANCA and is associated with crescentic glomerulonephritis, microscopic polyaangitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA) (formerly Churg-Strauss syndrome) [7], [12]. The gene discussed is MPO; the disease is eosinophilic granulomatosis with polyangiitis.