KH2 ES cells were retargeted with a "flp-in plasmid" containing a tetracycline responsive element (TRE or tetO) and a forced dimerization fusion cDNA of Jun lacking the stop codon and Fos connected by a flexible short polyGly-Ser peptide with a C-terminal FLAG-tag. A flpe-recombinase plasmid facilitated integration into the 3' UTR region.