The RET proto-oncogene is at the origin of one of the most interesting models of human disease caused by mutations in a receptor tyrosine kinase gene. Somatic rearrangements of RET are involved in the aetiology of a variable proportion of papillary thyroid carcinomas (PTC), the most common type of thyroid tumour whose prevalence is increasing in areas heavily exposed to radioactive fallout after the Chernobyl accident of 1986. Moreover, germline RET mutations are associated with the three variants of the inherited cancer syndrome known as multiple endocrine neoplasia type 2 (MEN2A, MEN2B and FMTC). Finally, RET mutations or heterozygous deletions of the whole gene cause the autosomal dominant form of Hirschsprung disease (HSCR), a congenital disorder of the enteric nervous system (ENS).