Germline gain-of-function mutations of the RET proto-oncogene are associated with three separate heritable endocrine neoplasia syndromes, all of which are associated with MTC: MEN2A also known as Sipple syndrome, multiple endocrine neoplasia type 2B (MEN2B) also known as the mucosal neuroma syndrome, and familial medullary thyroid cancer (FMTC). Here, RET is linked to multiple endocrine neoplasia type 2A.