Napsin is found in two isoforms, A and B, with highly homologous gene sequences. Napsin A has a molecular weight of 35.0 kDa and is also known as TA02. Napsin A is an aspartic proteinase which is expressed in the lung and involved in processing surfactant protein B (SP-B). It is also expressed in the kidney. This antibody may be a useful tool as a tumor marker for primary lung adenocarcinoma. Napsin expression correlates with the differentiation grade of lung adenocarcinoma.