Immunohistochemical analysis showed the tumor cells to be positive for cytokeratin-AE1/3, cytokeratin 7, synaptophysin, chromogranin A, calcitonin, and carcinoembryonic antigen, but negative for cytokeratin 20, Napsin A, Pax8, and p40, resulting in a diagnosis of metastasis of medullary thyroid carcinoma. This evidence concerns the gene PAX8 and medullary thyroid gland carcinoma.