A loxP site and a frt-flanked neomycin resistance gene were inserted upstream of exon 4 and a second loxP site was inserted downstream of exon 4. Flp-mediated recombination removed the selection cassette and cre-mediated recombination deleted exon 4. Deletion of exon 4 prevents the production of the exon4-containing splice form without reducing expression of the form lacking this exon.