RET and medullary thyroid gland carcinoma: Genotype-phenotype correlations are particularly important in MEN-2, and the 2015 Revision of the Guidelines for Management of Medullary Thyroid Cancer by the American Thyroid Association has classified RET variants into three risk categories on the basis of risk for MTC, which for the “high” and “highest” risk categories also correspond to those mutations for the greatest risk of PCC: "highest" (only the p.M918T mutation), "high" (p.A883F and p.C634 mutations), and "moderate" (other mutations) (63).