SPR deficiency results in neurologic deterioration due to severe dopamine and serotonin deficiencies in the central nervous system caused by a defect in BH4 synthesis. Clinically, affected individuals show an L-DOPA-responsive, diurnally fluctuating movement disorder usually associated with cognitive delay and severe neurologic dysfunction. BH4 is a required cofactor for the synthesis of the neurotransmitters dopamine and serotonin. BH4 is also a required cofactor for phenylalanine hydroxylase, but patients with SPR deficiency do not exhibit overt hyperphenylalaninemia. [From OMIM]