A neomycin resistance cassette replaced part of exon 3 and all of exon 4, eliminating expression of the endogenous short and long isoforms. RT-PCR and sequence analysis of brain tissue from homozygotes revealed a mutant mRNA was produced with exons 3 and 4 skipped, leading to a frame shift and premature stop codon in exon 5. This mutant transcript was predicted to produce a polypeptide that is only 9 amino acids long with no functional activity. No native protein was produced in homozygotes by Western blot analysis.