Histologic preparations should be reevaluated by IHC to guide the search for the primary tumor: TTF-1 (pulmonary or medullary thyroid carcinoma), CDX-2 (intestinal), PAX-8, histidine-decarboxylase (pancreatic), xenin (duodenal), gastrin (occult gastrinoma), and PP/glucagon (pancreatic) [38, 255]. The gene discussed is GAST; the disease is medullary thyroid gland carcinoma.