Homologous recombination in C57BL/6 mouse embryonic stem cells was used to replace exon 12 with the orthologous human PAH exon 12 sequence including 500 bp of the flanking genomic sequence of both 5' and 3' of exon 12. Additionally, exon 12 also contains the amino acid substitution at position 408 altering arginine to tryptophan (R408W; c.1222 C to T) of the phenylalanine hydroxylase (Pah) gene on chromosome 10. The construct also included a loxP-flanked neomycin selection cassette upstream of exon 12 which was subsequently removed by crossing to Cre deleter. (J:344302)