The targeting construct inserted upstream of exon 2 contain an FRT site, splice acceptor, IRES, beta-geo cassette, polyadenylation sequence, FRT site and loxP. An additional loxP site was inserted downstream of exon 2. Flp-mediated recombination removed the FRT-flanked beta-geo cassette and left exon 2 floxed. (J:211898)