The coding region of exon 1, which is specific to isoforms A and B (but not to isoform C), was replaced with a cassette containing IRES-GFPLacZ-loxP and an frt flanked hprt. Northern blot analysis of homozygous mutant brain tissue indicated an absence of A and B transcript and confirmed the presence of C transcript. The A and B isoforms were undetected by Western blot analysis. (J:83192)