Proteinase 3 (PR3) is unique among NSPs as it also represents the designated auto-antigen of granulomatosis with polyangiitis (GPA), an autoimmune disorder characterised by the inflammatory necrosis of small/medium-sized blood vessels and the common presence of anti-neutrophil cytoplasmic antibodies (ANCA) against PR35. Here, PRTN3 is linked to autoimmune disease.