A loxP site was inserted upstream of exon 22. An FRT-flanked neomycin resistance cassette with a 3' loxp site was inserted downstream of exon 23. Flp-mediated recombination removed the neomycin resistance cassette. Cre-mediated recombination removed exons 22 and 23. Western blot analysis confirmed the loss of the 190-, 270- and 480-kDa isoforms in the brain and the loss of the 190 kDa isoform but retention of the 150, 120 and 90 kDa isoforms in the kidney.