MEN2 is diagnosed in 5–10% of thyroid cancer patients and includes three conditions: MEN2A accounting for the vast majority of MEN2 cases, familial medullary thyroid carcinoma (FMTC) occurring in 10–20% of MEN2 patients, and MEN2B identified in approximately 5% of MEN2 patients. This evidence concerns the gene RET and familial medullary thyroid carcinoma.