SPR and hyperinsulinemic hypoglycemia, familial, 4: In addition, as PAH mediates the conversion of phenylalanine (Phe) to tyrosine (Tyr), hyperphenylalaninemia (HPA) is present in all BH4 deficiencies apart from autosomal dominant guanosine triphosphate cyclohydrolase I deficiency (AD-GTPCHD) and sepiapterin reductase deficiency (SRD) [1, 2].