MPO and granulomatosis with polyangiitis: While the clinical and pathological characteristics of granulomatosis with polyangiitis (GPA) and MPA were considered to be part of a single disease spectrum for several years, there is growing evidence that genetic associations, epigenetic control of major histocompatibility complex, and antigen function support the categorization of AAV by ANCA specificity (proteinase 3(PR3)-ANCA or myeloperoxidase (MPO)-ANCA) [6].