PAX8 is a transcription factor crucial to the organogenesis and development of the thyroid gland, urogenital tract, placenta and inner ear. In the thyroid, PAX8 is a master gene that regulates maintenance of the differentiated thyroid follicular cell phenotype, where it controls and activates the transcription of the main proteins responsible for the functional activity of follicular cells such as thyroglobulin, thyroperoxidase and sodium/iodide symporter. In the developing kidney PAX8 is important for renal vescicle formation. PAX8 regulates the expression of the WT1 gene. PAX8 appears to be currently the most sensitive and specific marker for renal cell carcinoma and ovarian non-mucinous carcinoma. Follicular and papillary thyroid carcinoma are virually always PAX8 positive (while anaplastic carcinoma is positive in most cases and medullary thyroid carcinoma negative). PAX8 is also found in almost all cases of ovarian serous, endometrioid, transitional and clear cell carcinoma (while mucinous carcinoma is positive in a minor number of cases), and endometrial carcinoma.