In 2003, a South Korean company called Maria Biotech announced its newest success: it had created mouse embryos with human cells in them.
The idea is that the mice could be born with human cells in all their tissues, and this would make them more accurate animal models for research. The problem came when a reporter asked whether there would truly be human cells in every tissue. (Yes.) Does that include human cells in ovaries and testes? (Presumably, yes.) So what happens if two of these mice get together, and a human sperm meets a human egg in the Fallopian tube of a mouse?
"That ended the project," says Kevin FitzGerald, a bioethicist at Georgetown University. The scenario described by the reporter was almost certainly impossible, but the incident represents some of the ethical questions around transplanting organs between species, or xenotransplantation.