The sequence encoding a FLPo/ERT2 fusion protein (optimized FLP recombinase fused to the tamoxifen-responsive mutated human estrogen receptor ligand-binding domain), SV40 polyadenylation site and a loxP-flanked PGK-neo cassette was cloned downstream of the human CMV enhancer/chicken ACTB promoter (CAG). This construct was targeted to the Gt(ROSA)26Sor locus in embryonic stem cells.