A loxP site was inserted into intron 2 and an FRT site flanked neomycin resistance gene cassette and a second loxP site into intron 4. This creates a conditional-ready allele with exons 3 and 4 floxed. Through cre-mediated recombination these two exons, coding for the active site of the enzyme, were deleted, also causing a frameshift in the downstream exons. This creates a null allele. (J:240765)