Medullary thyroid carcinoma (MTC) is a rare neuroendocrine malignancy of thyroid C-cells characterized by the secretion of several circulating biomarkers, including calcitonin (CT), procalcitonin (PCT), carcinoembryonic antigen (CEA), carbohydrate antigen 19-9 (CA 19-9), and pro-gastrin-releasing peptide (proGRP). The gene discussed is CEACAM5; the disease is medullary thyroid gland carcinoma.