Multiple endocrine neoplasia (MEN) types 1 and 2 are distinct dominantly inherited syndromes of cancer predisposition in man. MEN 1 involves the parathyroids, pituitary, and pancreatic islets; MEN 2 involves the thyroid C-cells, adrenal medulla and parathyroids. In some varieties of MEN 2 there are also developmental abnormalities of the autonomic nervous system of the gut. The MEN 1 predisposing gene has been mapped by linkage to chromosome 11q13, and it is likely that the gene will shortly be identified by positional cloning. The predisposing gene for MEN 2 has been shown to be the receptor tyrosine kinase ret. Mutations in different domains of ret are responsible for the different combinations of phenotypes seen in distinct clinical varieties of MEN 2 and in Hirschsprung disease.