Exon 2 was replaced with a modified exon 2 containing the untranslated region (2x) fused to a tetracycline transactivator (2x-tTA), a loxP site, an FRT-flanked neo cassette, a tetracycline responsive promoter (TRE) fused to the coding sequence of exon 2 (2y; TRE-2y), and a loxP site. Cre-mediated recombination removed the neo cassette and TRE-2y. (J:184364)