In the hereditary forms, the causative role of the germline RET variants has been clearly demonstrated, and there appears to be a strict correlation between genotype and phenotype, leading to different degree of risk related to the aggressiveness of the tumor and the clinical syndrome [familiar medullary thyroid cancer (FMTC), and type 2A and type 2B multiple endocrine neoplasia (MEN) (4)]. This evidence concerns the gene RET and multiple endocrine neoplasia.