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There's just one twist: the genes being mixed are animal and human. Already in Israel, researchers have put human embryonic stem cells into chick embryos. In Switzerland, they've created mice with human immune systems; in Minnesota, pigs with humanlike blood; in Nevada, sheep with near-human livers.
Many people find the whole idea morally repugnant, even if they have a hard time articulating just why this "yuck factor" bothers them. Bioethicists and others are also struggling to reason their way through the issue, weighing possible benefits and risks. The President's Council on Bioethics is studying human-animal chimeras. Next month, the National Academies of Sciences will release guidelines...