Deficiency in tetrahydrobiopterin (BH4) is a rare genetic disorder due to a defect in metabolism of amino acids. BH4 impairs the metabolism of enzymes resulting in hyper-phenylalanimia and a defect of neurotransmitter synthesis. Clinical manifestations are similar and usually indistinguishable from classic phenylketonuria (PKU). In most cases of BH4, progressive neurological symptoms appear between the ages of 2 and 12 months despite excellent dietary control. The syndrome may involve mental deterioration, intractable seizures, arrest in motor development, microcephaly, and occasionally abnormal movements.