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.1 TTF-1 is detected by nuclear staining seen in primary thyroid, lung adenocarcinomas and small cell carcinomas. Staining with TTF-1 antibody is useful for distinguishing between tumors of lung and non-lung origin. For unexpected staining seen in a metastatic tumor work up it is recommended to refer to literature.