Dihydropteridine reductase, also known as HDHPR and Quinoid dihydropteridine reductase, QDPR and DHPR, belongs to the short-chain dehydrogenases/reductases (SDR) family. QDPR exists as a homodimer. QDPR is part of the pathway that recycles a substance called tetrahydrobiopterin, also known as BH4 and tryptophan hydroxylases. The regeneration of this substance is critical for the proper processing of several other amino acids in the body. Tetrahydrobiopterin also helps produce certain chemicals in the brain called neurotransmitters, which transmit signals between nerve cells. Defects in QDPR are the cause of BH4-deficient hyperphenylalaninemia type C (HPABH4C) which is a rare autosomal recessive disorder and is lethal.