Nonketotic hyperglycinemia (NKH) is the inborn error of glycine metabolism defined by deficient activity of the glycine cleavage enzyme system (GCS), which results in accumulation of large quantities of glycine in all body tissues including the brain. Based on ultimate outcome NKH is categorized into severe NKH (no developmental progress and intractable epilepsy) and attenuated NKH (variable developmental progress and treatable or no epilepsy). The majority of children with NKH have onset in the neonatal period manifest as progressive lethargy evolving into profound coma and marked hypotonia; 85% have severe NKH and 15% attenuated NKH. Those with onset between two weeks and three months typically present with hypotonia; 50% have severe NKH and 50% attenuated NKH. Those with onset after age three months have attenuated NKH. Severe versus attenuated NKH is consistent within families, but the degree of developmental progress in those with attenuated NKH can vary. [from GeneReviews]