An FRT flanked neomycin resistance cassette with a 3' loxP site was inserted upstream of exon 3. An additional loxP site was inserted downstream of exon 4. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination removed exons 3 and 4. RT-PCR confirmed the absence of all three transcript variants in whole brain and trunk from E14.5 embryos. (J:223904)