The authors present the characteristic features of medullary carcinoma of thyroid (CMT) and underline the necessity to identify RET proto-oncogene that is the cause of hereditary transmission of CMT. Physiology of C cells and clinical syndromes are reported and the importance of a genetic screening in population at risk is emphasized; this test has shown to be reliable and easy to apply. They report their experience on techniques of amplification and restriction for RET proto-oncogene identification in relatives of patients with MEN or familial CMT syndromes. This study has allowed to recognize a population bearing the oncogene responsible of the disease and to achieve a correct prophylactic therapeutic management.