Hirschsprung's disease is a frequent congenital malformation of the hindgut. The existence of Hirschsprung's families favors the role of an autosomal dominant gene with a reduced penetrance. We have successively localized and identified the RET proto-oncogene as the gene responsible for familial Hirschsprung's disease. Interestingly, other mutations of the RET proto-oncogene have been described in inherited predisposition to endocrine cancers. This observation shows that, depending on their nature, mutations of the RET proto-oncogene could lead either to early developmental defects or to tumor predisposition.