TTF-1 has been the premier marker for lung adenocarcinoma. A new and promising marker, Napsin A, is expressed in type II pneumocytes and in adenocarcinomas of the lung (2). Studies have shown Napsin A to be more sensitive and specific than TTF-1 in lung adenocarcinomas and virtually negative in all squamous carcinomas. In other studies comparing TTF-1 and SP-A, Napsin A stained more tumor cells and a higher percentage of lung adenocarcinomas than either of these antibodies (1-3). Other studies have shown that when TTF-1 and Napsin A are used in combination, a higher sensitivity and specificity is achieved (4). A critical assessment is essential for correct diagnosis as patients with squamous carcinoma (SqCC) cannot receive Avastin due to a 30% mortality rate as a result of fatal hemoptysis (hemorrhaging). Therefore, when used in a panel with p63 and CK5, this unique multiplex antibody reagent of TTF-1 and Napsin A may aid in the analysis of poorly differentiated lung adenocarcinomas vs. squamous cell carcinomas in formalin-fixed paraffin-embedded tissues.