The RET gene codes for a tyrosine kinase receptor, expressed in neural crest derived cells playing a central role during embryogenesis. The RET proto-oncogene is responsible for medullary thyroid cancer and multiple endocrine neoplasia type 2. To date, more than 50 germline point mutations have been described. A specific correlation between genotype and phenotype is well recognized. Genetic testing is predictive of cancer onset, age at onset and biological aggressiveness. In recent years, the concept of codon-oriented prophylactic surgery has been introduced and three levels of risk have been identified on the basis of specific mutations. A review of the literature shows the excellent results of laboratory, genetic and clinical research that have made it possible to reduce medullary thyroid cancer-related mortality.