Multiple endocrine neoplasias (MEN) are syndromes inherited as autosomal dominant. The application of the techniques of molecular biology has made possible the identification of the genes causing MEN 1 and 2. The gene responsable for MEN 1 belongs to the family of tumor suppressor genes and encodes for a protein named MENIN whose function remains to be elucidated. The identification of mutant MEN 1 gene carriers who are at risk of developing this syndrome requires frequent biochemical screening for the development of endocrine tumors. MEN 2 is a consequence of mutations in the Ret proto- oncogene (c-Ret). This gene encodes for a tyrosine kinase receptor thought to play a role in the development of neural crest- derived tissue. Members of kindred with either MEN 2A or MEN 2B should be screened by direct DNA testing early in life for mutations in c-Ret. Those with the mutation should be advised to have thyroidectomy at five years of age in children with MEN 2A and earlier in children with MEN 2B . Some cases of sporadic MTC are actually MEN 2A or Familial MTC after c-Ret testing is done, therefore routine application of this test is recommended in all cases of apparent sporadic MTC (Rev Méd Chile 2000; 128: 811-20).