Phenylketonuria (PKU, OMIM #261600) is an autosomal recessive inborn error of metabolism in which the conversion of phenylalanine (Phe) to tyrosine by the phenylalanine hydroxylase (EC 1.14.16.1) is defective, resulting in partial or total inactivity of the conversion due to biallelic variants in the PAH gene [1]. The gene discussed is PAH; the disease is phenylketonuria.