With his careful compromise on embryonic stem-cell research, President Bush has put off - for now - a key ethical question of the 21st century.
But as federal funds begin to speed up the research, scientists will likely bump up against the limits Mr. Bush has set in a few years. Then the White House will again face very sensitive issues:
What is the moral status of a human embryo? Is it an object or a life? Is it defensible to destroy an embryo - even one that's due to be discarded - if its cells show potential to cure thousands?
Such questions represent the nation's first brush with a thicket of ethical issues that are likely to crop up as various biomedical technologies allow scientists to tinker with human life.
For the moment, the president's compromise seems to be placating many on both sides of the embryonic-stem-cell debate.
Scientists interested in the special cells, which have the ability to form themselves into any kind of cell in the body, can apply for federal funds for research.
Opponents can take comfort in the fact that such work won't destroy any more embryos than those that have already been destroyed to establish 60 stem-cell colonies or "lines."
Bush's plan, announced last week, should accelerate the research, says Harold Varmus, president of Memorial Sloan-Kettering Cancer Center in New York and former director of the National Institutes of Health. "There are at least several ... investigators who would want to work with these lines."
"The President has acted to save the lives that he could," David O'Steen, executive director of the National Right to Life Committee, said in a statement supporting the Bush plan. The group laments the destruction of embryos that has occurred, but says Bush had no way to undo that.
The immediate questions swirl around the stem-cell lines themselves. Research in this area has been carried out so quietly - because of its controversial nature and proprietary concerns - that even scientists in the field were surprised to hear that 60 stem-cell lines were available. What remains unclear is the quality of these lines.
IN theory, 60 useful stem-cell lines, propagating themselves indefinitely, would be enough to support the next phase of research, Dr. Varmus argues. Embryonic stem cells are so versatile that scientists hope to use them to fashion everything from more robust cancer treatments to replacement organs on demand. In practice, he adds, animal experiments suggest that truly useful stem-cell lines are tricky to develop and maintain.
Even if they are all useful, two challenges remain. First, it's not clear what private companies that developed the stem cells will do, once publicly funded scientists ask to use those cells. The independent organization that licenses the technology for the University of Wisconsin at Madison has promised to make its embryonic stem cells widely available to researchers. But other stem-cell providers may add restrictions.
Secondly, scientists will probably eventually need more than 60 lines to ensure that an organ or tissue raised from stem cells isn't rejected by a patient's body. Bush's compromise "will give researchers something to do for the time being," says Adam Katz, an adult-stem-cell researcher at the University of Virginia in Charlottesville. "But it may become limiting at some point." Thus, the White House may well have to revisit whether to fund research involving newly destroyed embryos.
Of course, the president's decision leaves private research firms free to carry on their own stem-cell work. But the private sector often relies on government to fund initial research. Thus, federal money often determines how much research gets done.
Consider the funding of adult stem cells, a related field.
Last year, the National Institutes of Health forked over $147 million to study human adult stem cells and another $79 million for similar research on animals. (Adult stem cells are far less controversial, because nothing dies in extraction.)
The US private sector, by contrast, is kicking in some $10 million to $15 million a year on embryonic-stem-cell research, estimates Michael West of Advanced Cell Technologies.
Not surprisingly, entrepreneurs are jumping into adult stem cells, even though embryonic stem cells are thought to offer far more flexibility.
"Commercial opportunities for uses of adult stem cells are here now," says John Glushik of Intersouth Partners, a Durham, N.C., venture capital firm.
Bush has yet to make clear how much federal money he wants to spend on embryonic stem cells. Some scientists have estimated that totals could run to $100 million. Others are skeptical of that figure.
"I'm quite pessimistic," says James Olds, director of George Mason University's Krasnow Institute for Advanced Study. He predicts that, despite a compelling need, "the amount of research on stem cells ... is likely to stay the same or decrease somewhat in the public sector."