A targeting vector was designed to insert a Cre/ERT2 fusion gene, an SV40 polyA signal, and an frt-flanked neo-polyA cassette into the translation start site of Gdf5 gene. This construct was electroporated into C57BL/6-derived embryonic stem (ES) cells. Correctly targeted ES cells were injected into recipient blastocysts and chimeric mice were bred with white C57BL/6J-congenic ROSA26::FLPe knock-in females to remove the neo selection cassette and to originate the colony. (J:234264)