Napsin A has special functions in normal alveolar epithelial and plays a role in the protein hydrolysis of the surfactant precursor. Napsin A is reported to be mainly expressed in lamellar body of II type pulmonary alveolus cell, secondary lysosomes of alveolar macrophage, terminal bronchiole,respiratoryepithelium of respiratory bronchioles ,plasma cell, lung normal lymphocytes and normal renal tubular epithelial cells, as well as expressed in normal spleen slightly.However, the research reported that Napsin A is expressed in 90% primary lung cancer approximately.