An in-frame termination codon followed by a floxed neomycin resistance gene cassette was inserted at the first codon of the fourth exon, which encodes a subtype-specific region of isoform B. The absence of isoform B protein expression was confirmed by western blot analysis on brain extracts. Additionally, the levels of A isoform protein expression are decreased to 20% of wild-type. (J:143248)