TH and dystonia 5: However, DRD can still be caused by defects in other enzymes that are involved in dopamine biosynthesis, such as tyrosine hydroxylase [TH, encoded by TH gene (OMIM 191290)], sepiapterin reductase [SR, encoded by the SPR gene (OMIM 182125)], pyruvoyl-tetrahydropterin synthase [PTPS, encoded by the PTS gene (OMIM 612719)], pterin-4a-carbinolamine dehydratase [PCD, encoded by the PCBD gene (OMIM 126090)], and dihydropteridine reductase [DHPR, encoded by the QDPR gene (OMIM 612676)].