Anti-neutrophil cytoplasmic antibodies (ANCA)-associated vasculitis (AAV) is a heterogeneous group of small-vessel vasculitides, characterized by the presence of antibodies binding to myeloperoxidase (MPO) and proteinase-3 (PR3) found in neutrophil granules. Here, MPO is linked to anti-neutrophil cytoplasmic antibody-associated vasculitis.