Proteinase 3 (PR3) is one of four serine protease homologues in the azurophilic granules of neutrophils and granules of monocytes. The primary function of these neutrophil-derived serine proteases is thought to be degradation of extracellular proteins at sites of inflammation, but excessive or prolonged proteolytic activity may cause harmful effects in the body. PR3 is the epitope of anti-neutrophil cytoplasmic antibodies (ANCAs) of the cANCA (cytoplasmic subtype) class, a type of antibody frequently found in the disease Wegeners granulomatosis; a necrotizing vascultis syndrome. In active Wegener the prevalence of this antibody is almost 90%, decreasing to 30-40% during remission. Furthermore, PR3 is overexpressed in a variety of acute and chronic myeloid leukemia cells. Cytotoxic T lymphocytes specific for a PR3-derived peptide have been shown to specifically lyse leukemia cells that overexpress PR3. Therefore, the PRESENCE of anti-PR3 antibodies correlates strongly with Wegeners granulomatosis and are much less frequently found in Churg-Strauss Syndrome and Microscopic Polyangiitis. Other related tests: Neutrophil Cytoplasmic Antibodies, Serum; and Myeloperoxidase (MPO) Antibodies, Serum.