Test Details: Immunohistochemical testing is carried out on biopsy/resected tissue specimen. This testing helps to identify specific antigens (proteins) within tissue sections utilizing an antigen-specific antibody .Thyroid transcription factor (TTF-1 also known as NKX2-1 or thyroid-specific enhancer-binding protein) is a homeobox-containing transcription factor that regulates the development, differentiation and gene expression of the thyroid gland (follicular and parafollicular C cells). TTF-1 plays also an active role in the regulation of development and transcriptional activity of the lung and central nervous system (diencephalon). In adult thyroid gland, TTF-1 is expressed in both follicular and parafollicular cells and controls the synthesis of different thyroid hormones and thyrotropin receptor. In normal lung, TTF-1 is strongly expressed in type II alveolar cells, Clara bronchiolar cells, and, in a lesser degree, in the epithelial cells of tracheal mucosa. TTF-1 is widely used as a specific and sensitive marker for the majority of bronchopulmonary adenocarcinomas and pulmonary small cell carcinoma in addition to follicular, papillary, and medullary thyroid carcinomas . A lesser degree of expression is found in large cell carcinoma of the lung and undifferentiated thyroid carcinoma, Non-pulmonary small cell carcinoma of different locations, subset of extrahepatic cholangiocarcinomas, glial-ependymal and choroid plexus tumours, pituicytoma, granular cell tumour of the sellar region, meningeal tumours.