An FRT-flanked neomycin resistance cassette with a 5' loxP site was inserted upstream of exon 3. An additional loxP site was inserted downstream of exon 4. Cre-mediated recombination removed the selection cassette along with exons 3 and 4. RT-PCR and Western blot analysis confirmed absence of mRNA and protein expression in the brain, liver, and heart from homozygous mutant mice.