A 5' loxP site was inserted in intron 1 at position 628 nucleotides 5' of exon 2, and the second 3' loxP site together with an frt-flanked PGK-neo selectable marker was inserted 140 nucleotides 5' of exon 7. Flp-mediated recombination removed the selection cassette, leaving exons 2-6 floxed.