Arginine:glycine amidinotransferase (AGAT) deficiency in a newborn: early treatment can prevent phenotypic expression of the disease.

Arginine:glycine amidinotransferase deficiency is a treatable inborn error of creatine synthesis, characterized by mental retardation, language impairment, and behavioral disorders. We describe a patient in whom arginine:glycine amidinotransferase was diagnosed at birth and treated at 4 months with creatine supplementation. In contrast with his 2 older sisters, he had normal psychomotor development at 18 months.