British regulatory authorities have ruled that they are prepared to permit the creation of hybrid embryos in which human DNA would be inserted into enucleated animal eggs for the purpose of generating embryonic stem cells for research (http://www.hfea.gov.uk/en/1581.html).
A principal goal of such research is to create patient-specific embryonic stem cells by removing most of the DNA from the egg cell of an animal (such as a rabbit or cow) and replacing it with the nucleus from a human cell. Human egg cells are in short supply—most human embryonic stem cell research currently uses human eggs left over from fertility treatment—in part because the process of donating egg cells requires a prospective donor to undergo hormone treatment and poses some medical risk.
Stephenson J. Hybrid Embryos. JAMA. 2007;298(15):1752. doi:10.1001/jama.298.15.1752-c