A loxP site was inserted upstream of exon 3' and an FRT site flanked neomycin resistance gene cassette and loxP site were inserted downstream of exon 3'. The neo cassette was removed through subsequent flp-mediated recombination. The floxed exon 3' was deleted through cre-mediated recombination. This deletion specifically knocks out the specific deltaN isoform that uses an alternative exon 3 that lacks the acidic transactivation domain sequence.