Antileukoproteinase, also known as secretory leukocyte protease inhibitor (SLPI), is an enzyme that in humans is encoded by the SLPI gene. SLPI is a highly cationic single-chain protein with eight intramolecular disulfide bonds. It is found in large quantities in bronchial, cervical, and nasal mucosa, saliva, and seminal fluids. SLPI inhibits human leukocyte elastase, human cathepsin G, human trypsin, neutrophil elastase, and mast cell chymase. X-ray crystallography has shown that SLPI has two homologous domains of 53 and 54 amino acids, one of which exhibits anti-protease activity ( C-terminal domain). The other domain ( N-terminal domain) is not known to have any function.