The targeting construct inserted tetracycline response element (TRE or tetO) and a loxP site into intron 1 and fused EGFP, FRT-flanked neomycin resistance cassette and an additional loxP site to the ATG translation initiation site in exon 2. Cre-mediated recombination removed EGFP and FRT-flanked neomycin resistance cassette.