RET and multiple endocrine neoplasia type 2B: 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).