For a fleeting time, a day or two at most, all things are possible for an extraordinary group of cells that form part of a newly created embryo. Known as embryonic stem cells, they have the capacity to grow into any sort of tissue the body will need--nerves, blood, heart, bone and all the rest. And then they start to do just that, abandoning their unlimited promise in order to do something useful with their lives. Scientists have long believed, however, that embryonic stem cells could be terrifically useful in their unspecialized state as well, not only as a source of...
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