Glycine is a simple, nonessential amino acid, although experimental animals show reduced growth on low-glycine diets. The average adult ingests 3 to 5 grams of glycine daily. Glycine is involved in the body's production of DNA, phospholipids, and collagen, and in the release of energy. Glycine levels are effectively measured in plasma in both normal patients and those with inborn errors of glycine metabolism (http://www.dcnutrition.com/AminoAcids/). Nonketotic hyperglycinaemia (OMIM: 606899) is an autosomal recessive condition caused by deficient enzyme activity of the glycine cleavage enzyme system (EC 2.1.1.10). The glycine cleavage enzyme system comprises four proteins: P-, T-, H- and L-proteins (EC 1.4.4.2, EC 2.1.2.10, and EC 1.8.1.4 for P-, T-, and L-proteins). Mutations have been described in the GLDC (OMIM: 238300), AMT (OMIM: 238310), and GCSH (OMIM: 238330) genes encoding the P-, T-, and H-proteins respectively. The glycine cleavage system catalyzes the oxidative conversion of glycine into carbon dioxide and ammonia, with the remaining one-carbon unit transferred to folate as methylenetetrahydrofolate. It is the main catabolic pathway for glycine and it also contributes to one-carbon metabolism. Patients with a deficiency of this enzyme system have increased glycine in plasma, urine, and cerebrospinal fluid (CSF) with an increased CSF:plasma glycine ratio. Glycine is also found to be associated with carbamoyl phosphate synthetase deficiency, iminoglycinuria, maple syrup urine disease, phenylketonuria, propionic acidemia, sarcosinemia, and tyrosinemia I, which are inborn errors of metabolism.