LoxP sites flank exons 3-5 and a puromycin resistance cassette flanked by FRT sites was inserted into intron 5. Flp-mediated recombination removed the puromycin selection marker. Cre-mediated recombination removed exons 3-5, removing a part of the protease domain and generated a frameshift from exons 2 to 6. (J:234481)