Although not pathognomonic, a hallmark feature of AAVs is the presence of ANCA directed against neutrophil granule components, namely: proteinase 3 (PR3-ANCA), which is more frequently observed in GPA, and myeloperoxidase (MPO-ANCA), which is more commonly found in MPA, EGPA, and renal-limited vasculitis [2]. The gene discussed is MPO; the disease is eosinophilic granulomatosis with polyangiitis.