Human-animal cytoplasmic hybrid embryos, mitochondria, and an energetic debate.

Scientists are seeking permission to generate human embryonic stem cells to study disease by introducing human genetic material into an animal oocyte. This has raised ethical questions that centre on whether the entities being generated are actually human. The answer to these questions will determine how this area of research will be regulated and whether such work will be legal. The function of the extra-nuclear mitochondrial genome lies at the heart of these issues and forms the focus of this commentary.