Myeloblastin (EC 3.4.21.76, leukocyte proteinase 3, leukocyte proteinase 4, Wegener's granulomatosis autoantigen, proteinase PR-3, proteinase-3, PMNL proteinase) is an enzyme. This enzyme catalyses the following chemical reaction hydrolysis of proteins, including elastin. This enzyme is present in polymorphonuclear leukocyte granules.