Thyroid transcription factor-1 (TTF-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 not reactive. TTF-1 is detected in primary lung adenocarcinomas and small cell carcinomas. It is absent in mesotheliomas, colon cancer and breast cancer. TTF-1 can be very useful when used in a panel with CK7, CK20, CDX2, Villin and CA125 antibodies.