MTC is sporadic in origin in 60 to 75 % of all patients with MTC, while the others exhibit germline mutations in the RET proto-oncogene, namely patients with multiple endocrine neoplasia type 2A (MEN2A), multiple endocrine neoplasia type 2B (MEN2B), and familial MTC syndrome (FMTC). Here, RET is linked to familial medullary thyroid carcinoma.