This is not cloning to make babies, but to create medicine.
"It's important to point out that this is not a project to clone a person, but rather to make cells that can be used for therapy in the sense of making tissues for transplantation," Colorado State University cloning expert George Seidel told CBS Radio News.
However, it's sure to revive international controversy over whether to ban all human cloning, as the Bush administration wants, or to allow this "therapeutic cloning" that might eventually let patients grow their own replacement tissue.
Embryonic stem cells are the body's building blocks, cells from which all other tissue types spring. They're present in an embryo only days after conception and are ethically sensitive because culling stem cells destroys the embryo.
Scientists have used therapeutic cloning to partially cure laboratory mice with an immune system disease. And they know how to cull stem cells from human embryos left over in fertility clinics.
But attempts to clone human embryos — so the resulting stem cells would be genetically identical to the patient who needs them — have failed until now.
"It's a first step. The success rates are very low. It's inefficient, it's expensive, but in the long run this will enable making tissues that can be transplanted but will not be rejected," said Seidel.
Scientists from Seoul National University say they succeeded largely because of using extremely fresh eggs donated by South Korean volunteers and gentler handling of the genetic material inside them.
The lead scientist, veterinary cloning specialist Woo Suk Hwang, will unveil the research Thursday at a meeting of the American Association for the Advancement of Science. Details will be published in the journal Science.
It's elegant work that provides long-anticipated proof that human therapeutic cloning is possible, said stem-cell researcher Dr. Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Mass.
Still, "it's not of practical use at this point," Jaenisch cautioned.
Years of additional research are required before embryonic stem cell transplants could be considered in people, he stressed.
"I would say we're decades away. Things may progress faster than I think, but there are all kinds of impediments," agreed Seidel.
But it's sure to renew debate over whether all forms of human cloning should be banned. The U.S. House of Representatives last year voted to do that, but the Senate stalled over whether there should be an exception for some research.
U.S. scientists almost universally want a ban on cloning for reproduction, because the high rate of birth defects in cloned animals shows the technique is too dangerous.
But the South Korean research is "one tiny step closer to some medical use. It would be a wise thing to support," said Laurie Zoloth, a Northwestern University bioethicist. "It is clearly time — now that it is more tangible — to set in place a process where we can have some kinds of experiments supported and some things banned."
The United Nations recently postponed a decision on what kinds of human cloning to ban. The United States is pushing for a total ban; Britain is leading the call for cloning for medical experiments to be left unhindered.
The Seoul researchers collected 242 eggs from 16 unpaid volunteers. Each woman also donated some cells from her ovary.
Using the same process as is used to clone animals, they removed the gene-containing nucleus of each egg and replaced it with the nucleus from the donor's ovarian cell.
Chemicals jump-started cellular division, resulting in 30 blastocysts, early-stage embryos that contain a mere 100 cells. From those, they harvested just one colony of stem cells — a small success rate.
But those stem cells were a genetic copy of the donor, and began forming muscle, bone and other tissues in test tubes and when implanted into mice, the Seoul team reported.
Now, the team is studying how to direct which tissues those cells form, said Woo, who pledged in an e-mail interview to make the new cell line available to other interested scientists.
But Jaenisch lamented that many U.S. scientists couldn't work with the new cell line. Bush administration policy forbids any federally funded research on stem cells from embryos destroyed after Aug. 9, 2001.