One loxP site was inserted in the 5'UTR of exon 1 while a second loxP site, followed by an FRT flanked neomycin cassette in reverse orientation, was inserted in intron 7. The selection cassette was subsequently removed by transient flp recombinase expression leaving exons 1-7 flanked by loxP sites. Transient cre expression then excised the floxed exons.