The targeting construct inserted into intron 1 an FRT site, F3 site, loxP site, lox511, splice acceptor, beta-galactosidase gene, polyadenylation sequence, loxP site, lox511 site, FRT site, and F3 site. Flp-mediated recombination inverted the cassette allowing for normal gene splicing until crossed with a cre recombinase expressing gene. (J:333184)