Donor females are mated, blastocysts are harvested, and 10-15 ES cells are injected into each blastocyst. Eight to ten blastocysts are then implanted into a uterine horn of each pseudopregnant recipient. This happens when integration of the transgene is delayed until after the first cell division. There can also be multiple insertion events at different genomic loci and at different times. Thus, a single founder can be mosaic for one insertion site but not the other.
Transgenic animals are a powerful tool for studying gene function and testing drugs.
Many human genetic diseases can be modeled by introducing the same mutation into a mouse or other animal. Although similar genetic manipulations can be performed in tissue culture, the interaction of transgenes with proteins, hormones, neurotransmitters, and other components of an intact organism provides a much more complete and physiologically relevant picture of the transgene's function than could be achieved in any other way. With the development of transgenic livestock and plants, new uses for this technology have become apparent, some of potentially great economic and medical value.