RET and multiple endocrine neoplasia type 2B: In MEN 2, clinical heterogeneity has been noted due to mutations in several codons in the RET gene: the great majority of MEN 2A (now changed simply to MEN2) are associated with a mutation at codon 634, exon11 which codes for the extra-cellular domain of RET, while for MEN 2B (now MEN3) the dominant mutation lies in codon 918, exon 16 which codes for part of the intra-cellular domain.