A target vector was designed to insert an âfl-NLS-lacZ-fl-mCherry-FRT-neo-FRTâ cassette (containing ATG start codon and nuclear localization signal sequence 5â of lacZ reporter gene with 3â BGH polyA, flanked by 2 LoxP sites to allow reporter exchange to a downstream ATG-mCherry-pA cassette by Cre recombination, followed by a FRT-flanked neomycin selection cassette) in-frame at the ATG start codon in exon 2 of the gene Tet1. The targeting construct was electroporated into C57BL/6 mouse embryonic stem (ES) cells and selected in G418. The transgenic KI mouse strain was created by blastocyst injection of one correctly targeted clone. The strain has been bred to Flp-deleter mice to remove the neo cassette, resulting in one remaining FRT site downstream of the fl-NLS-lacZ-fl-mCherry cassette. The final modification is designated as Tet1-fl-lacZ-fl-mCherry.