The identification of germline point mutations in different domains of the RET proto-oncogene in inherited human diseases, namely Multiple Endocrine Neoplasia type 2A and 2B (MEN2A and MEN2B), familial or sporadic medullary thyroid carcinoma (MTC) and Hirschsprung's disease (Donis-Keller et al, 1993; Mulligan et al, 1993; Edery et al, 1994; Hofstra et al, 1994; for review Eng, 1999), confirms that this gene plays a critical role in the differentiation and growth of specific cell lineages of neural crest origin (i.e. thyroid C-cells). This evidence concerns the gene RET and multiple endocrine neoplasia type 2.