Napsin is a pepsin-like aspartic protease with a molecular weight of approximately 38 kDa. Napsin A is closely related to Napsin B. Napsin A is expressed as a single chain protein in type II pneumocytes and in adenocarcinomas of the lung and kidney. The high specificity expression of Napsin A in adenocarcinomas of lung is useful to distinguish primary lung adenocarcinomas from adenocarcinomas of unknown orgin. Therefore Napsin A is a promising marker for the diagnosis of primary lung adenocarcinoma.

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