A loxP site was inserted upstream of exon 2. An FRT-flanked neomycin resistance cassette with a 3' loxP site was inserted downstream of exon 4. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination removed exons 2 through 4. Western blotting confirmed loss of protein in brain, spleen, and thymus tissues from homozygous mutant mice.