Thyroid transcription factor, also called thyroid specific enhancer binding nuclear protein (38 kDa), that regulates transcription activity of thyroid (thyroglobulin, thyroid peroxidase, sodium-iodide transport protein, calcitonin and MHC class I) and lung (surfactant proteins A, B and C, Clara cell secretory protein). The expression of TTF1 is confined to follicular epithelial cells and the C-cells in the thyroid, and to the type II pneumocytes and the Clara cells in the lung. In tumor diagnostics TTF1 distinguish primary (TTF+) vs. metastatic (usually TTF1-) lung carcinoma (LCa), pulmonary adenoca (TTF1+) from squamous cell Ca (usually TTF1 -), pleural lung Ca (TTF1+) vs.mesothelioma (TTF1 - ) and pulmonary small cell Ca (TTF1+) vs. Mercel cell Ca (TTF - ). Mucinous lung adeno ca is usually TTF1 negative.