Exon 6 was flanked with loxP sites and an FRT site flanked marker and selection double cassette was inserted into intron 5. The double cassette consists of a beta galactosidase (LacZ) gene cassette and a neomycin resistance gene cassette, separated by a loxP site. The marker-selection cassette was deleted through flp-mediated recombination, leaving a conditional ready allele with a floxed exon 6. (J:236749, J:237888)