RET and multiple endocrine neoplasia type 2: Germline RET mutations are responsible for the development of autosomal dominant multiple endocrine neoplasia types 2A and 2B (MEN2A and MEN2B) and familial medullary thyroid carcinoma (FMTC) (35), (36), (37), all of which confer an increased risk (~100%) of developing medullary thyroid carcinoma (MTC).