TTF1 and pleural mesothelioma: TTF-1 IHC is widely used by pathologists for the distinction of primary pulmonary adenocarcinoma—typically TTF-1 positive—from pulmonary squamous cell carcinoma, metastatic adenocarcinoma to the lung, and pleural mesothelioma which are TTF-1 negative in most cases [7, 13, 41].