Abstract.Sixteen FMTC, detected mutations affect one of five cysteine residues in the extracellular region adjacent to the transmembrane segment of the RET protein [2,4,5]. In MEN 2B, a mutation at codon 918 causing the substitution of threonine for methionine of the tyrosine kinase domain of the RET protein was detected in almost every case [3,6,7]. A missense mutation at codon 768 in the tyrosine kinase domain of the RET proto-oncogene was recently described in a family with FMTC, which does not have a cysteine codon mutation [8]. The same mutation was also detected in 40% of sporadic MTCs which do not have codon 918 mutations.With respect to sporadic pheochromocytomas, we detected codon 918 mutations in 31% of sporadic pheochromocytomas [9]. In the present study, we analyzed sporadic pheochromocytomas and pheochromocytomas with neurofibromatosis 1 (NF1), MEN 2A, or MEN 2B for mutations of codon 768 of the RET proto-oncogene.