Napsin is a pepsin-like aspartic proteinase connected with the maturation of surfactant protein B. There are two closely related napsins, napsin A and napsin B. Napsin A is expressed as a single-chain protein. Immunohistochemical studies revealed high expression levels of napsin A in the human lung and kidney but low expression in the spleen. Napsin A has expressed in type II pneumocytes and adenocarcinomas of the lung. The high specificity expression of napsin A in adenocarcinomas of the lung is helpful to distinguish primary lung adenocarcinomas from adenocarcinomas of other organs.