RET is a gene that is involved in cell growth in the thyroid, parathyroid and adrenal glands.
Everyone has two RET genes (one from their mother, and one from their father). If one of the genes is not working, this is known as having a faulty RET gene, or having a RET mutation. People with a faulty RET gene are known to have Multiple Endocrine Neoplasia type 2.