TGT KO mice are viable and largely asymptomatic but do show symptoms similar to those associated with phenylketonuria (PKU) in that they are deficient in tyrosine production from phenylalanine and BH4 levels are significantly decreased in the plasma, while the BH4 oxidation product dihydrobiopterin (BH2) is elevated [27]. Here, QTRT1 is linked to phenylketonuria.