We want to derive ES-cells from the mouse strain SPRET/Ei. This way, we want to expand the toolbox of techniques and applications to genetically manipulate mammals. We will do this by isolating preantral follikels from young SPRET-Ei females and let them mature in vitro. The matured oocytes will then be fertilized in vitro with SPRET/Ei sperm and the rsulting zygotes will be kept in culture till the blastocyst stage. These blastocysts will then be used to derive SPRET/Ei ES-cells.