Exon 4 was replaced with an exon containing a phenylalanine to isoleucine mutation at position 77 (F77I). This point mutation confers drug insensitivity to zolpidem. A loxP site was inserted in the upstream intron and a frt-flanked neomycin cassette followed by another loxP site was inserted in the downstream intron. Chimeric mice were mated with a Flp deleter strain to remove the neomycin cassette. Genomic sequencing of homozygous mice confirmed the presence of the F to I mutation.