TTF-1 positivity is often used to determine thyroid and lung tumors. If it is suggestive of thyroid disease, the prognosis is generally good; if it is suggestive of lung disease, the prognosis is generally poor. 1. Thyroid: In adult tissues, TTF-1 is mainly distributed in the follicular cells of the thyroid gland, and is positively expressed in papillary thyroid carcinoma. TTF-1 positivity is different in benign and malignant thyroid tissues, with more expression in normal thyroid and benign adenomas, less in papillary and follicular thyroid carcinomas, and no expression in undifferentiated carcinomas. Since TTF-1 positivity is often expressed in benign tumors in thyroid diseases, the prognosis is generally better. 2. Lung: TTF-1 positivity is generally expressed in fetal lung tissue, adult type II alveolar epithelium. And, among lung tumors, most small cell carcinomas and adenocarcinomas of the lung, most atypical neuroendocrine carcinomas of the lung, and a few large cell undifferentiated carcinomas of the lung can be positively expressed. Lung squamous carcinoma, typical carcinoid tumor and other cases are rarely expressed. Since TTF-1 positivity is often expressed in malignant lung disease, the prognosis in such cases is generally poor.