Exons 3-5 were flanked by loxP sites and a FRT-flanked neomycin resistance gene and F3-flanked puromycin resistance gene were inserted into intron 2 and intron 5, respectively. Flp-mediated recombination removed the selection makers and cre-mediated recombination removed exons 3-5, generating a frameshift from exons 2 to 6 and a premature stop codon in exon 6.