Exons 2 and 3 was flanked by loxP sites (size of loxP-flanked region: 3.9 kb). Positive selection markers was flanked by FRT (Neomycin resistance - NeoR) and F3 (Puromycin resistance - PuroR) sites and were inserted into intron 1 and intron 3, respectively. Flp-mediated recombination removed the selection cassette. (J:314544)