The differential diagnosis of mediastinal neuroendocrine neoplasms also includes metastasis from other sites, such as medullary thyroid carcinoma (positive for calcitonin and monoclonal CEA), lung carcinoids (which may express TTF1), gastroenteropancreatic low- to intermediate-grade neuroendocrine tumors (which may express CDX2), and pancreatic neuroendocrine tumors (which may express PAX8 and progesterone receptor) [59,60,61,62,63,64]. This evidence concerns the gene TTF1 and neuroendocrine neoplasm.