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Dihydropteridine reductase (QDPR), also named as DHPR and HDHPR, is an essential enzyme in the hydroxylating system of the aromatic amino acids, since it catalyses the regeneration of tetrahydrobiopterin (BH4), the natural cofactor of phenylalanine, tyrosine, and tryptophan hydroxylases, from the quininoid-dihydrobiopterin produced in these coupled reactions(PMID:8326489). The QDPR protein is active as a dimer, with a subunit Mr of 26 kDa(PMID:7627180). This protein belongs to the short-chain dehydrogenases/reductases (SDR) family. Defects in QDPR are the cause of BH4-deficient hyperphenylalaninemia type C (HPABH4C)(PMID:11153907).