This gene encodes a member of the peptidase A1 family of aspartic proteases. The encoded preproprotein is proteolytically processed to generate an activation peptide and the mature protease. The activation peptides of aspartic proteinases function as inhibitors of the protease active site. These peptide segments, or pro-parts, are deemed important for correct folding, targeting, and control of the activation of aspartic proteinase zymogens. The encoded protease may play a role in the proteolytic processing of pulmonary surfactant protein B in the lung and may function in protein catabolism in the renal proximal tubules. Napsin A plays a role in pneumocyte surfactant processing. In normal tissue, Anti-Napsin A specifically labels type II pneumocytes in adult lung and epithelial cells in kidney tissues. 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.