Napsin A is a pepsin-like aspartate protease with a molecular weight of about 38 kDa. Napsin A is closely related to Napsin B. Napsin is expressed as a single-chain protein in type II pneumocytes and in adenocarcinomas of the lung and kidney. The high specificity of Napsin A in lung adenocarcinomas is useful in distinguishing such primary adenocarcinomas from adenocarcinomas of unknown origin.

[1] Ueno T, Linder S and Elmberger G (2003): Aspartic proteinase napsin is a useful marker for diagnosis of primary lung adenocarcinoma. Br J Cancer. 88(8):1229-33.
[2] Brasch F, Ochs M, Kahne T et al. (2003): Involvement of napsin A in the C- and N-terminal processing of surfactant protein B in type-II pneumocytes of the human lung. J Biol Chem. 278(49):49006-14.
[3] Hirano T, Auer G, Maeda M et al. (2000): Human tissue distribution of TA02, which is homologous with a new type of aspartic proteinase, napsin A. Jpn J Cancer Res. 91(10):1015-21.
[4] Tatnell PJ, Powell DJ, Hill J et al. (1998): Napsins: new human aspartic proteinases. Distinction between two closely related genes. FEBS Lett. 441(1):43-8.