An inborn error of metabolism resulting from a deficiency of hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8) with increased conversion of glycine to uric acid with excessive purine synthesis and hyperuricemia. Patients are normal at birth but begin to show hypertonicity at about 4 months and irritability and other neurological symptoms become apparent during the second year of life, the child becoming gradually more aggressive and self-destructive, banging his head, biting the lower lip and, less commonly, the upper lip, cheeks, fingers, and hands, sometimes using the fingers to mutilate his ears and nose. Associated disorders include spastic cerebral palsy, choreoathetosis, renal uric acid calculi, gouty tophi, and uric acid nodules. Mental retardation is common but normal intelligence occurs in some cases.