Phenylalanine hydroxylase (PAH) deficiency (OMIM #261600) is caused by autosomal recessive variants in the phenylalanine hydroxylase (PAH) gene and leads to an impaired degradation of the amino acid phenylalanine (Phe) to tyrosine and, as a consequence, to elevated concentrations of Phe in blood (1). The gene discussed is PAH; the disease is phenylketonuria.