Phenylketonuria (PKU, OMIM 261600) is a rare genetic disorder (with an incidence of 1 in 10,000 births in Europe [1]) characterised by a deficiency of the hepatic enzyme, phenylalanine hydroxylase (PAH, EC 1.14.16.1), responsible for the conversion of the essential amino acid phenylalanine (Phe) into tyrosine. The gene discussed is PAH; the disease is phenylketonuria.