Napsin A is an aspartic proteinase that resembles pepsin. Itis expressed in lung and kidney adenocarcinomas as well astype II pneumocytes. Research has indicated that Napsin Ais superior to TTF-1 in terms of both sensitivity andspecificity. In non-small cell lung cancer, Napsin Ademonstrated a greater specificity (94.3%) foradenocarcinoma than TTF-1 (76.1%). Napsin A, in contrastto TTF-1, is positive in certain RCCs. According to a numberof studies, 83% to 90.7% of initial lung adenocarcinomastested positive for napsin A. In contrast to primary lungcancer, which exhibits granular cytoplasmic staining intumour cells, other neoplastic tissues, such as ovarianmalignancies, have modest expression and distinct stainingpatterns. Napsin A stained a greater proportion of lungadenocarcinomas and more tumour cells than either TTF-1or SP-A in studies comparing the two antibodies. Napsin Ais a promising marker for the diagnosis of primary lungadenocarcinoma since it can be used to differentiate it fromadenocarcinoma of unknown origin.