A loxP site was inserted into intron 1 and an FRT site flanked neomycin resistance gene cassette and a second loxP site into intron 2. The neo cassette was removed through subsequent flp-mediated recombination, leaving exon 2 floxed. A G>A substitution was engineered in exon 2, but the exon was deleted through subsequent cre-mediated recombination.