Napsin is an aspartic proteinase similar to pepsin that belongs to the A1 group of the AA category of proteinases. There are two napsins that are very similar, which are napsin A (NAPSA) and napsin B (NAPSB). Napsin A plays a role in the processing of propeptide pulmonary surfactant protein B (proSP-B) in the lungs. In healthy tissue, Napsin A is found in type II pneumocytes in the lungs and in the proximal tubules of the kidneys. Napsin A serves as an important marker for lung adenocarcinoma.