Now that hybrid embryos have effectively been legalised in the UK, doubts about whether they are possible have surfaced in the media. The UK's fertility regulator, the Human Fertilisation and Embryology Authority, recently backed requests by scientists to create so-called "cybrids", formed by fusing human DNA with an enucleated cow or rabbit egg.

However, Dr Robert Lanza, of Advanced Cell Technology, an American company which has been working on hybrid embryos for some time, says that his efforts to do this have been unsuccessful when two species were quite different. "They grow to the 16-cell stage, then just before going on to become blastocysts, they block," he told New Scientist. He suspects that this happens because the animal mitochondria in the cytoplasm stops communicating with the human genetic material.

However, Dr Stephen Minger, who has applied for a licence to use the technique, is planning to send researchers to China to learn how to do it. Researchers at Shanghai Second Medical University claim that they created these hybrids four years ago. He is upbeat about British science. "Just because it hasn't been done, it doesn't mean we shouldn't try it," he told New Scientist.