CRISPR/cas9 endonuclease mediated genome editing is used to introduce a reversed loxP site to intron 3 and an FRT-flanked PGK-neomycin resistance cassette and reversed loxP site is placed in intron 4. Flp-mediated recombination removed the FRT-flanked neo cassette leaving exon 4 floxed. (J:257337)