RET and medullary thyroid gland carcinoma: By comparison, the vast majority of MTC in the pediatric population are hereditary, the result of germline RET mutations resulting in multiple endocrine neoplasia (MEN) type 2A syndrome (90–95% of childhood MTC), MEN type 2B, or familial medullary thyroid carcinoma (FMTC) [6,27,28].