A missense mutation has been identified in the phenylalanine hydroxylase (PAH) gene of a Danish patient with hyperphenylalaninaemia (HPA). An A-to-G transition at the second base of codon 414 results in the substitution of Cys for Tyr in the mutant PAH protein. In in vitro expression studies, the Tyr414-to-Cys414 mutant construct produced a protein which exhibited a significant amount of normal PAH enzyme activity, which is consistent with both in vitro and in vivo measurements of PAH activity in HPA patients. Population genetic studies reveal that this mutation is present on 50% of mutant haplotype 4 chromosomes in the Danish population. Together with the previously reported codon 158 mutation, these two mutant alleles comprise over 90% of all mutant haplotype 4 chromosomes in the Northern European population. Thus, two allele-specific oligonucleotide probes can detect most mutant haplotype 4 chromosomes in Northern Europe.