Hyperphenylalaninaemia (HPA) is diagnosed as an abnormal elevation in blood phenylalanine levels and is usually caused by autosomal recessive mutations in the genes encoding for phenylalanine hydroxylase enzyme (in the case of phenylketonuria, PKU) or for the enzymes involved in 6R-tetrahydrobiopterin (6R-BH4) biosynthesis or regeneration (in the case of BH4 deficiency).