Napsin

Napsin A, also known as NAPSA, is a lysosomal aspartic protease encoded by the NAPSA gene. It is expressed predominantly in type II alveolar pneumocytes of the lung, where it participates in the maturation of surfactant proteins, particularly the processing of pro-surfactant protein B to its active form.

In clinical practice, napsin A is widely used as an immunohistochemical marker to support the diagnosis of primary lung adenocarcinoma. Tumors of pulmonary origin frequently show positive napsin A staining, whereas squamous cell carcinomas, either primary or metastatic, are typically negative. Napsin A is commonly interpreted in combination with other markers, such as TTF-1, to improve diagnostic accuracy and to help distinguish primary lung adenocarcinoma from metastases from other sites.

Like all immunohistochemical markers, napsin A is not perfect. A subset of lung adenocarcinomas may be napsin A-negative, and a minority of non-pulmonary tumors can show napsin A positivity. Therefore, napsin A results are used alongside morphologic assessment and a broader panel of markers, within the appropriate clinical and radiologic context.

Overall, napsin A is a clinically important protein in pathology, serving both a biological role in pulmonary surfactant processing and a practical role as a diagnostic aid in distinguishing lung adenocarcinoma from other malignancies.