Napsin is an aspartic protease connected with maturation of surfactant protein B. Napsin-A is localized to the cytosol of alveolar macrophages and type 2 pneumocytes but is also present in the epithelial cells of the kidney as well as the lungs. Used together with TTF-1 (thyroid transcription factor-1) which is used routinely for lung adenocarcinoma staining, napsin-a can be used for increased sensitivity and specificity in primary lung tumors and in metastases (1, 3, 4). Napsin-A is also frequently expressed in renal cell carcinomas. Antibody to napsin-A provides a useful tool for lung and renal carcinoma study.