A construct was designed to insert an internal ribosome entry site (IRES), cre recombinase gene, Frt site, RNA pol II-Neo-HSVtk-polyA cassette, and a second Frt site downstream of the progesterone receptor (Pgr) transcriptonal stop codon. The construct was electroporated into 129S/SvEv-derived embryonic stem (ES) cells. Correctly targeted ES cells were injected into recipient blastocysts.
Chimeric males were bred with C57BL/6J females to establish the colony. At some point, mutant mice were bred with GtROSA-FLPe mice (undisclosed genetic background) to remove the frt-flanked neo cassette.