Even though serum calcitonin and CEA are biomarkers for medullary thyroid carcinoma (Hamada et al. 1976), some recent studies report that those biomarkers are of no clinical use (Bockhorn et al. 2004), while other studies add progastrin-releasing peptide, which is considered to be a specific marker for small cell lung carcinoma, as an additional marker for the diagnosis and monitoring the response to therapy in patients with medullary thyroid carcinoma (Ide et al. 2001). This evidence concerns the gene CEACAM5 and medullary thyroid gland carcinoma.