Multiple endocrine neoplasia type 2B (MEN 2B), or the mucosal neuroma syndrome, is an autosomal dominant hamartoneoplastic syndrome. Features include multiple mucosal neuromas, phaeochromocytoma, medullary thyroid carcinoma, and Marfanoid body habitus with a characteristic dysmorphic facies. The gene responsible is the receptor tyrosine kinase (RET) proto-oncogene on chromosome 10. The mutational spectrum of MEN 2B is remarkably narrow, with over 95% of cases being caused by a single methionine to threonine substitution in the intracellular tyrosine kinase domain. Recent biochemical evidence suggests that this mutation alters the substrate specificity of intracellular signal transduction.