In the other 25% of patients, MTC is hereditary, mainly in the context of multiple endocrine neoplasia type 2A (MEN2A) (90–95%) and type 2B (MEN2B) (5–10%), which are autosomal dominant syndromes caused by a germline mutation in the REarranged during Transfection (RET) proto-oncogene [5]. This evidence concerns the gene RET and multiple endocrine neoplasia type 2.