Napsin A is a pepsin-like aspartic proteinase that belongs to the peptidase A1 family and plays a role in pneumocyte surfactant processing. Napsin A is closely related to Napsin B, expressed mainly in the lung and kidney and which is involved in the correct folding, targeting, and control of aspartic proteinase zymogens. Napsin A expression has been indicated in type II pneumocytes and adenocarcinomas of the lung and kidney. In abnormal tissues, Napsin A is strongly positive in over 80% of primary lung adenocarcinomas and 79% of renal cell carcinoma by immunohistochemistry. Napsin A is a useful marker for lung adenocarcinoma. The combined use of Napsin A and thyroid transcription factor-1 (TTF-1) improves the sensitivity and specificity for identification of pulmonary adenocarcinoma.