GPA is characterized by antigen-specific IgG4, IgG3 or IgG1 against proteinase 3 (PR3; surface antigen) or myeloperoxidase (MPO; intracellular protein) in neutrophils and monocytes, increased levels of IgG4+ plasma cells, fibrosis and sometimes elevated serum IgG4 levels (49–52). The gene discussed is MPO; the disease is granulomatosis with polyangiitis.