Phenylketonuria (PKU) is an inborn error of metabolism caused by more than 1200 different variants of the PAH gene, many of them leading to a reduced enzymatic activity of the encoded phenylalanine hydroxylase (PAH) enzyme (PAHvdb, http://www.biopku.org, last accessed on 7 February 2022). The gene discussed is PAH; the disease is phenylketonuria.