MPO and granulomatosis with polyangiitis: Recent studies suggest that genetic background, risk of disease relapse, prognosis, and co-morbidities are closely linked to the ANCA serotype, e.g., proteinase 3 (PR3)-ANCA (or c-ANCA) and myeloperoxidase (MPO)-ANCA (or p-ANCA), associated with the disease phenotypes GPA or MPA.