Preferentially cleaves the synthetic substrate H-D-Leu-Thr-Arg-pNA compared to tosyl-Gly-Pro-Arg-pNA. Expressed abundantly in the epithelial cells of the airways, including trachea, esophagus and fetal lung. Scarce in adult lung. Expressed at low levels in placenta, pancreas, prostate and thyroid gland. Belongs to the peptidase S1 family.