Phenylalanine hydroxylase deficiency or phenylketonuria (PKU) is a rare, autosomal recessive disorder characterised by an inability to metabolise the essential dietary amino acid phenylalanine (Phe) into tyrosine (Tyr), due to a mutation in the genes that code for the phenylalanine hydroxylase (PAH) enzyme [1]. Here, PAH is linked to phenylketonuria.