QDPR and hyperinsulinemic hypoglycemia, familial, 4: Generally, inborn errors of metabolism comprise a heterogeneous group of disorders, caused by mutations in one of the genes encoding enzymes involved in biosynthesis (guanosine triphosphate cyclohydrolase [GTPCH] or 6-pyruvoyl-tetrahydropterin synthase [PTPS]) or regeneration (pyruvate carboxylase deficiency [PCD] or dihydropteridine reductase [DHPR] of BH4.