Even as supporters of human embryonic stem cell research are reeling from last week's sudden cutoff of federal funding, another portentous landmark is quietly approaching: the world's first attempt to carefully test the cells in people.

Scientists are poised to inject cells created from embryonic stem cells into some patients with a progressive form of blindness and others with devastating spinal cord injuries. That's a welcome step for researchers eager to move from the laboratory to the clinic and for patients hoping for cures.

But beyond being loathsome to those with moral objections to any research using cells from human embryos, the tests are worrying many proponents: Some argue that the experiments are premature, others question whether they are ethical, and many fear that the trials risk disaster for the field if anything goes awry.

"We desperately need to know how these cells are going to perform in the human setting," said John Gearhart, a stem cell pioneer at the University of Pennsylvania. "But are we transplanting cells that are going to cause tumors? Will they will stay where you put them and do what you want them to do?"

Supporters of these privately funded, government-sanctioned tests, including patients' advocates, bioethicists and officials at the companies sponsoring them, are confident the research has been exhaustively vetted. The Food and Drug Administration has demanded extensive experiments in the laboratory and on animals to provide evidence that the cells are safe enough to test in people and hold great promise.

"We're very optimistic," said Thomas Okarma, president and chief executive of Geron Corp. of Menlo Park, Calif., which after years of delay received a green light in July from the FDA to study patients partially paralyzed by spinal cord injuries. "If we're right, we'll revolutionize the treatment of many chronic diseases."

But some researchers fear that the stakes jumped even higher with the federal judge's blocking federal funding. If patients are hurt by the cells -- or even if there's no hint the cells help -- that could be a devastating blow just as scientists are scrambling for funding from private foundations and benefactors. They cite the case of Jesse Gelsinger, whose 1999 death from a gene therapy experiment set that once highly touted field back years.

"There's a lot of angst around these trials," said Evan Snyder, director of the stem cell program at the Sanford-Burnham Medical Research Institute in San Diego. "There's going to be this perception that if the cells do not perform well, the entire field will be illegitimate."

Most of the apprehension focuses on the Geron trial. Safety worries -- most prominently fears that the cells could cause tumors -- prompted the FDA to repeatedly demand additional data from Geron, including most recently assurance cysts that developed in mice injected with the cells posed no threat.

"We jumped through a lot of hoops to convince a lot of audiences," Mr. Okarma said. "No one wants another Jesse Gelsinger."

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