The cloning of the phenylalanine hydroxylase gene and cDNA has potentially allowed the complete characterization of patients with phenylketonuria and already many mutations have been defined. Parents of patients now have the option of prenatal diagnosis. The 18 mutations defined so far indicate enormous heterogeneity not only within particular populations but also between populations. These mutations give little indication as to the locations of the amino acid residues important in enzyme function but one-third of the mutations are in exon 7 which may be indicating the importance of the region coded by this exon in the protein.