A FRT-flanked neomycin resistance cassette driven by beta-actin replaced exon 18, which is specific to the 180 kDa isoform of the protein. Immunoblot analysis of brain tissues from homozygous mutant mice did not detect the 180 kDa isoform in neonatal and adult tissues, and detected the 140kDa isoform in neonatal tissues, but not in adult tissues. (J:37891)