A loxP site was inserted into intron 2 and an FRT site flanked neomycin resistance gene cassette and a second loxP site into intron 3. The neo cassette was removed through subsequent flp-mediated recombination. Exon 3 was deleted through subsequent cre-mediated recombination. Exon 3 codes for the mature peptide, so this creates a null allele. (J:244834)