Mutations in the RET extracellular domain at various cysteine residues (C609, C618, C620, and C634) have been detected in MEN2A MTC patients, whereas mutations in the RET intracellular tyrosine kinase domain (mostly RET M918T) have been detected in MEN2B MTC patients [9,10]. Here, RET is linked to medullary thyroid gland carcinoma.