TTF-1(Thyroid transcription factor-1) is a member of the NKX2 family of homeodomain transcription factors. It is expressed in epithelial cells of the thyroid gland and the lung. Nuclei from liver, stomach, pancreas, small intestine, colon, kidney, breast, skin, testes, pituitary, prostate, and adrenal glands are unreactive. TTF-1 is detected in primary lung adenocarcinomas and small cell carcinomas and is absent in colon and breast carcinomas. Staining with TTF-1 antibody is useful for distinguishing between tumors of lung and non-lung origin.