Napsin A is a human aspartic proteinase found primarily in type II pneumocytes, alveolar macrophages and in the renal proximal tubules. Napsin A is more sensitive and specific in detecting lung adenocarcinomas than TTF-1. Differentiation of NSCLC into histologic types is important with the emergence of new targeted therapies. Literature suggest that a panel consisting of Napsin A, TTF-1, p63 and CK 5/6 is useful in distinguishing lung adenocarcinoma from squamous cell carcinoma.