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Researchers in New York are reporting that for the first time, they've used cloning techniques to successfully create human embryos in the lab. Now, the goal is not to make a human baby. As NPR's Joe Palca reports, they're after embryonic stem cells that can be used for medical therapies.

JOE PALCA, BYLINE: Before I can explain why this new work is important, I have to give you a bit of background, so bear with me on this. It's a bit complicated. Cloning is a technique that allows you to make what is essentially a genetic copy of a living organism. Ever since Scottish scientists made the cloned sheep, Dolly, there's been a nagging question: could you clone a human being? No one has done so, and there's virtually universal agreement that it would be wrong even to try.

But there's another use for cloning, not to make a new individual, but to stop at the embryo stage and use that cloned embryo to make embryonic stem cells. Many scientists believe that stem cells made this way could revolutionize medicine. Here's an analogy that I think will help you understand why they think that.

Let's say you know you're going in for surgery in a few months to have your hip replaced. The surgeon says you may lose blood during the surgery, and recommends that you donate a pint of your own blood in advance in case you need it during the operation. The advantage of donating your own blood is that there's no chance your immune system will reject that blood if it's put back into you.

It's solving that rejection problem that makes stem cells derived from cloned embryos attractive. Many researchers see a day when they can make embryonic stem cells tailored specifically to you. Although several research teams have tried, no one has succeeded in making a cloned human embryo.

But Dieter Egli and his colleagues at the New York Stem Cell Foundation have taken a step forward. First off, they tried to make a cloned embryo basically the same way Dolly was made. They took a human egg, removed its genetic material and replaced it with the genetic material from an adult skin cell. The adult cell has the genetic instructions for the clone, and in animals, the egg can go about the business of becoming an embryo with that adult DNA. When I spoke with Egli about his research, he said he wanted to revisit that method.

DR. DIETER EGLI: It was clear from animals that the animal eggs that have been tried do have that ability. But no, that's not necessarily has to translate to human cells.

PALCA: Human embryos made this way didn't grow, so he tried modifying the Dolly technique. He left in the egg's genetic material instead of removing it, and then added the genetic material from the adult cell. That did the trick. The embryo started growing.

But there was still a problem. Leaving in the egg's DNA meant each cell in the embryo now had the wrong number of chromosomes, three sets instead of the two that are normally there. Transplanting stem cells derived from these embryos as a medical therapy wouldn't work. Our bodies just wouldn't know what to do with three sets of chromosomes. But at least Egli has answered the question of whether it's possible to make a cloned human embryo.

EGLI: The clear answer of our paper to this is yes, and this is really the major finding of the paper.

PALCA: Egli says this result gives him the confidence to keep working with his approach to see if they can make it work with the right number of chromosomes.

DR. GEORGE DALEY: This isn't the gold ring, but it's a major step forward.

PALCA: George Daley is a stem cell researcher at Children's Hospital in Boston. Even if Egli perfects his cloning techniques, there are still many people morally opposed to this kind of research. Daley says there's a new kind of stem cell called an iPS cell that may prove to be just as good as embryonic stem cells. But work on iPS cells is just getting started.

DALEY: The jury is still out as to whether the iPS cells are going to be the gold standard in the long run.

PALCA: So for now, Daley thinks work with cloned human embryos should continue. Joe Palca, NPR News, Washington.

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