An autosomal recessive metabolic disorder caused by deficiencies in the mitochondrial GLYCINE cleavage system; an enzyme system with four components: P-, T-, H-, and L-proteins. Deficiency of the P-protein is the most prevalent form. Neonatal and juvenile presentations have been reported. Neonatal onset is more common and may be fatal. Clinical features include SEIZURES; hypotonia; APNEA; and COMA. When the illness presents in childhood there tends to be an associated progressive DEMENTIA accompanied by extrapyramidal signs. (Menkes, Textbook of Child Neurology, 5th ed, p46; Jpn J Hum Genet 1997 Mar;42(1):13-22).