Thyroid transcription factor-1 (TTF-1) is a tissue specific transcription factor expressed in epithelial cells of the thyroid and lung, as well as in certain areas of the brain. Recent investigations have shown that TTF-1 is also commonly present in a wide variety of thyroid and lung tumors. Because of its restrictive expression, TTF-1 is a useful immunohistochemical marker in the diagnosis of tumors of thyroid or lung origin.