(1) dHPR-deficient form; an inherited disorder in which there is an absence or deficiency of dihydropteridine reductase (DHPR); this results in impaired regeneration of tetrahydrobiopterin, causing an elevation in phenylalanine levels; (2) gTP-CH form; an inherited disorder in which there is a deficiency of guanosine triphosphate cyclohydrolase, an enzyme used in the biosynthesis of tetrahydrobiopterin; (3) 6-PTS form; an inherited disorder in which there is a deficiency of 6-pyruvoyltetrahydropterin synthase, an enzyme that participates in the biosynthesis of tetrahydrobiopterin. SYN: nonclassical phenylketonuria.