Thyroid transcription factor 1 (TTF-1) is a nuclear protein with a molecular weight of 38-40 kDa. It is the basic material for thyroid differentiation and the regulation of thyroglobulin secretion. It can promote the transport of thyroid peroxidase and iodine / sodium. TTF-1 is mainly distributed in type II alveolar epithelium, respiratory epithelium and Clara cells of thyroid and lung in adult tissues, but it is not expressed in type I alveolar epithelium. It is mainly used for the diagnosis of small cell lung cancer, the differential diagnosis of primary lung adenocarcinoma from metastatic adenocarcinoma, and the differential diagnosis of lung cancer from malignant mesothelioma and paraganglioma of pleura.