The targeting construct inserted upstream of exon 5 a loxP site and downstream of exon 5 an FRT-flanked neomycin resistance cassette and 3' loxP site. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination removed exon 5. Western blot analysis demonstrated that protein expression is undetectable in the brain of homozygous mutant mice.