A targeting vector containing a mini gene locus with a codon improved cre recombinase cDNA with a translational stop codo, splice donor site, artificial intron and an exon encoding the SV40 polyadenylation signal followed by an FRT-flanked PGK-neo cassette in the opposite orientation was introduced into exon 1 of Sox17 by homologous recombination in TBV2 ES cells. Germline chimeras were generated and the neo cassette was excised by crossing these with Flpe mice.