Photo: © Science

VARIOUS STAGES: Human embryonic stem cell colonies, shown here in different states of development, sometimes include a core of undifferentiated cells surrounded by a margin of differentiated cells, such as the small colony at right in figure B.
Gene therapy researchers call them the "ultimate target." Tissue transplant specialists refer to them as the "Holy Grail." Stem cells, perhaps because they play such a fundamental role in the developmental chain, tend to draw hyperbole. But hyperbole resulting from two recently published reports on successful cultures of different lines of immortalized, pluripotent human stem cells in vitro, has made some of the researchers involved cringe. Unwelcome responses to the findings include media reports that would have scientists soon growing organs in petri dishes.

James A. Thomson, an associate research veterinarian at the University of Wisconsin's Regional Primate Research Center in Madison, led a group that derived stem...

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