TTF1 and lung carcinoma: Napsin-A, a functional aspartic proteinase expressed in the cytoplasm of healthy lung parenchyma, is more sensitive than TTF-1 in distinguishing primary lung carcinoma from other adenocarcinomas.[25–28] Indeed, it is a useful additional immunohistochemical staining to TTF-1 for determining the origin of metastatic adenocarcinomas.