Thyroid Transcription Factor 1 (TTF-1), also known as NK2 homeobox 1 (NKX2-1), is a member of the NKx2 family of homeodomain transcription factors. It binds and activates the promoter of thyroid specific genes such as thyroglobulin, thyroperoxidase, and thyrotropin receptor. TTF-1 is important in the maintenance of thyroid differentiation and lung development. TTF-1 is present in diencephalon, lung, and thyroid. TTF-1 positive cells are found in Type II pneumocytes and Clara cells in the lung. Anti-TTF-1 stains thyroid and thyroid-derived tumors, and is therefore used for distinguishing lung adenocarcinoma from germ cell tumors, malignant mesothelioma, and metastatic carcinomas from organs other than the thyroid. It is also useful for distinguishing small cell lung carcinoma from lymphoid infiltrates, and pulmonary from non-pulmonary adenocarcinomas in malignant effusions. The ability to distinguish between pulmonary and non-pulmonary adenocarcinomas is particularly useful in identifying tumors that have metastasized to the brain.