We know that embryonic stem cells can differentiate into any tissue of the human body; might they therefore also be able to treat diseases like Parkinson's, Alzheimer's and diabetes? In principle, this ability to differentiate into blood, muscle or neural tissue may make embryonic stem cells the gold standard for replacing bad tissue with good. But some antiabortion advocates, rankled that these cellular chameleons come from embryos, call for a categorical ban on funding this research.

In 1996 Congress forbade the use of federal funds for research that would involve destroying human embryos. Last year, however, the National Institutes of Health issued guidelines, supported by the Clinton administration, that would allow embryonic stem cell research to continue as long as the harvesting step was not conducted with federal monies.

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