KH2 ES cells containing Gt(ROSA)26Sortm1(rtTA*M2)Jae and Col1a1tm13(neo/hygro*)Jae were retargeted with a "flp-in plasmid" containing a tetracycline responsive element (TRE or tetO) and FLAG-tagged Fos cDNA sequence. A flpe-recombinase plasmid to facilitated integration into the 3' UTR region.