A targeting vector containing GFP-Cre/ERT2 cassette (GCE) and FRT site flanked Kanamycin/neomycin selection cassette was used to disrupt the exon containing the ATG translation start-site. The construct was electroporated into 129P2/OlaHsd derived E14Tg2a.4 embryonic stem (ES) cells. No green fluorescence was detected by direct fluorescence microscopy; however, immunohistochemical analysis revealed GFP expression in E18.5 embryos. (J:159099)