A tetrahydrobiopterin (BH4)-deficient hyperphenylalaninemia that is characterized by hyperphenylalaninemia, depletion of the neurotransmitters dopamine and serotonin, and progressive cognitive and motor deficits and that has_material_basis_in homozygous or compound heterozygous mutation in the QDPR gene, which encodes an enzyme involved in the salvage pathway for BH4, on chromosome 4p15.