Leon kass, chairman of President Bush's bioethics council, calls it "the wisdom of repugnance". Others know it simply as the "yuck factor" - that visceral feeling that there is something wrong, even if you cannot say what. That gut feeling, Kass argues, is telling you that a moral boundary has been crossed and it is time to turn back. The yuck factor has made more frequent appearances in recent years as biotechnology has advanced. Most recently it has emerged in discussions of human-animal chimeras -living entities that have both human and animal cells. One researcher, for example, wants to create a mouse with a brain composed entirely of human neurons. That might sound like science fiction, and for now it is. But scientists have already created similar human-animal amalgamations, and there are many more on the drawing board (see page 39).
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