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. Germ line cre-mediated recombination was used to remove the neo cassette. The absence of isoform B protein expression was confirmed by western blot analysis on brain extracts. Expression of isoform A protein is unaffected. (J:143248)