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Napsin A is an aspartic proteinase involved in the proteolytic processing of surfactant precursors in normal alveolar epithelium. It is predominantly expressed in lamellar bodies of type II pneumocytes, alveolar macrophages, respiratory epithelium of terminal and respiratory bronchioles, plasma cells, and in subsets of lymphocytes in the normal lung, as well as epithelial cells of renal tubules. Weak expression is also noted in spleen. Clinically, Napsin A expression is observed in most primary lung adenocarcinomas and may also be seen in renal carcinoma.