RET and familial medullary thyroid carcinoma: In particular, germ-line mutations in the RET gene are responsible for the hereditary tumour syndrome (i.e., multiple endocrine neoplasia type 2, MEN 2) which includes three subgroups, MEN 2A, MEN 2B, and familial medullary thyroid carcinoma (FMTC), depending on the tissue involved.