The patient is classified as having GPA if having histological evidence of GPA or antineutrophil cytoplasmic antibodies (ANCA) specific for proteinase 3 (PR3-ANCA) or myeloperoxidase (MPO-ANCA), and surrogate markers that suggests granulomatous disease of the upper or lower respiratory tract, for example chronic rhinosinusitis (CRS) for more than 3 months [6]. The gene discussed is MPO; the disease is chronic rhinosinusitis.