TH and dystonia 5: DRD results from deficient 3,4-dihydroxyphenylalanine (L-DOPA) synthesis, either through insufficient BH4 production or from defective tyrosine hydroxylase (TH; EC 1.14.16.2; tyrosine 3-monooxygenase), the enzyme that catalyzes the conversion of tyrosine (Tyr) to L-DOPA, the precursor of CAs (Figure 1).