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Brain-Specific Serine Protease 4 (BSSP-4) is a serine protease that preferentially cleaves the synthetic substrate H-D-Leu-Thr-Arg-pNA compared to tosyl-Gly-Pro-Arg-pNA. BSSP-4 is expressed abundantly in the epithelial cells of the airways, including trachea, esophagus and fetal lung, but scarce in adult lung and expressed at low levels in placenta, pancreas, prostate and thyroid gland. BSSP-4 belongs to the peptidase S1 family and related to trypsin, referentially hydrolyzing substrates after arginine and lysine residues. However, BSSP-4 is less susceptible to inhibition by common trypsin inhibitors such as aprotinin, α1-antitrypsin and secretory leukocyte protease inhibitor. BSSP-4 efficiently converts pro-urokinase- type plasminogen activator to its mature, active form.