The causative role of germline RET mutations in MEN 2 syndromes and the strict correlation between genotype and phenotype was clearly demonstrated by the study of the International RET Consortium that collected and published very important data about the RET mutations and the clinical and pathological features of 477 kindred affected by MEN 2A, MEN 2B and FMTC [78]. Here, RET is linked to multiple endocrine neoplasia type 2A.