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From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish.

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And I'm Melissa Block. Scientists are reporting a major breakthrough today. For the first time, they have cloned human embryos that can produce stem cells. It's a long-sought step that's considered crucial for using stem cells to treat diseases. But as NPR's Rob Stein reports, it's also highly controversial.

ROB STEIN, BYLINE: Ever since human embryonic stem cells were discovered, scientists have had high hopes for them. That's because they can turn into any kind of cell in the body. So researchers like John Gearhart, of the University of Pennsylvania, think they could eventually be used to cure all sorts of illnesses.

JOHN GEARHART: Diabetes, various types of neurological diseases, heart disease - it's across the board.

STEIN: But these cells have always been the focus of intense debate. Very early human embryos are destroyed to get them, and some people think that's immoral. Despite that, scientists have been trying to make embryonic stem cells that are perfect genetic matches for patients. Gearhart says that way, their bodies won't reject the cells.

GEARHART: It's been a holy grail that we've been after for years.

STEIN: But year after year, every attempt ended in failure.

GEARHART: Along the way, there were several prominent investigators saying that this could probably never be done.

STEIN: But Shoukhrat Mitalipov, of the Oregon Health and Science University, never gave up. He did it in mice, then he did it in monkeys. And in this week's issue of the journal Cell, Mitalipov describes how he finally did it in humans.

SHOUKHRAT MITALIPOV: In my laboratory, it's actually one of the main interests - is to understand what's the factors that's responsible for this magic transformation of any cells into young, embryo-like cells.

STEIN: To achieve this magic transformation, here's what Mitalipov did. He recruited women who were willing to provide eggs for his research. Then he removed most of the DNA from each egg. Next, he took genes from other people's skin cells, and placed those genes into the eggs. Finally, he searched and searched for just the right way to stimulate each egg so that it would develop into an embryo.

Just the right combination of chemicals, and an electric pulse.

MITALIPOV: We had to find the perfect combination of the electric pulses that they'd need, plus concentration of this chemical.

STEIN: As it turned out, that perfect combination included something surprising: caffeine.

GEORGE DALEY: The Starbucks experiment, I guess.

STEIN: That's George Daley, a leading stem cell scientist at Harvard.

DALEY: The caffeine allowed the success. This little change in the cocktail was what allowed the experiment to really, ultimately succeed.

STEIN: Succeed in generating embryos containing stem cells, healthy embryonic stem cells capable of becoming any kind of cell in the body. He even got them to become heart cells that in a laboratory dish could pulse like a heartbeat.

DALEY: This is a huge scientific advance. Once we learn how to harness their power, they could actually be used in cell replacement therapies.

STEIN: Therapies that could one day treat many diseases. But the work is also drawing a lot of criticism for lots of reasons. First of all, the Oregon researchers compensated women financially to donate eggs for the experiments, something long considered ethically questionable. And even more than that, the research involved creating and destroying embryos.

For people who believe an embryo has the same moral standing as a human being, that's morally repugnant. Daniel Sulmasy is a professor of medicine and bioethics at the University of Chicago.

DANIEL SULMASY: Deliberately creating a human being with the sole purpose of destroying that human being, even for a good purpose, I'm of the school that thinks that that's morally wrong no matter how much good could come of it.

STEIN: And there's even bigger concerns. The techniques Mitalipov used are basically the same that were developed the same as Dolly the sheep. So that raises the specter that they could be used to try to clone a human being. After all, Mitalipov created embryos that reached the same stage of development as embryos that are routinely used to make babies at IVF clinics.

SULMASY: This raises serious problems because it is the first actual human cloning. We already know there are people out there who are itching to be able to be the first to bring a cloned human being to birth. And I think it's going to happen.

STEIN: Now, Mitalipov dismisses those concerns. He says the embryos he created are not the equivalent of a human being because they were not fertilized naturally. And his experiments with monkeys indicate it's unlikely that they could ever develop into a healthy baby.

MITALIPOV: The procedures that we developed actually are very efficient to make stem cells, but it's unlikely that this will be very useful for kind of reproductive cloning.

STEIN: Other researchers agree and stem cell pioneer John Gearhart says the possible benefits of the research outweigh the concerns.

GEARHART: I think that where you have a situation where you have patients that are in need of some type of therapy and none is available, where you can improve their quality of life tremendously through this kind of technology, I personally believe that it is ethical to use material like this.

STEIN: The scientists acknowledge that it will be years before anyone knows whether this step will actually turn into treatments that might help patients. In the meantime, it's clear that the intense debate over embryonic stem cells is far from over. Rob Stein, NPR News.

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