Highest expression in lung and tongue. Also expressed in brain, colon, heart and liver. Isoform 1 is the predominant form in tongue whereas both isoforms are expressed in similar amounts in lung. At the cellular level, expression is confined to epithelial cells within the cleft of the circumvallate papillae extending into the ducts of the minor salivary glands, the respiratory epithelium of the nasal cavity and tear gland ducts. Belongs to the peptidase S1 family.