Researchers in South Korea (search) for the first time have cloned a human embryo and then culled stem cells from it, marking an important step toward one day growing patients' own replacement tissue to treat diseases.

The experiment is sure to revive controversy over human cloning (search), both in the United States and internationally.

This is not cloning to make babies. Instead it's called therapeutic cloning (search), in which embryos that are the genetic twins of a particular patient are grown in a test-tube to supply master stem cells (search) that can grow into any tissue -- without being rejected by that patient's immune system.

The technique offers the potential of breakthrough treatments for diabetes, Parkinson's and other diseases, but any therapy is years away from being tested in people.

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, offering the potential of cell therapy but not patient-specific treatment.

But attempts at cloning a human embryo in the stem-cell quest have failed until now.

Scientists from Seoul National University report they succeeded -- thanks, they say, to using extremely fresh eggs donated by South Korean volunteers and finding a gentler way of handling the genetic material inside them.

The report appears in Friday's edition of the journal Science.

It's elegant work that provides long-anticipated proof that the technique is possible using human cells, said stem-cell researcher Dr. Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Mass.

"That's an important point to prove," he said.

Still, "it's not of practical use at this point," Jaenisch said, stressing that years of additional research are required.

For one thing, the cloning technique still doesn't work well: The Seoul team collected 242 eggs, from which they succeeded in cloning 30 blastocysts -- early-stage embryos containing a mere 100 cells. From those, they harvested just one colony of stem cells.

Still, it's likely to renew debate over whether all forms of human cloning should be banned. In Congress, the House last year voted to do that, but the Senate stalled over whether there should be an exception for research of this type.

Internationally, the United States is pushing for a United Nations ban of all human cloning, too. The U.N. General Assembly recently postponed a decision. There is almost universal support for a global ban of reproductive cloning, but Britain and a number of other countries want cloning for medical experiments left unhindered.

Cloning aside, Jaenisch lamented that most U.S. scientists won't be able to experiment with the Seoul researchers' new stem-cell line. Culling stem cells from embryos kills them, and President Bush has forbidden any federally funded research on stem cells from embryos destroyed after Aug. 9, 2001 -- making the South Koreans' recently developed line too new.

Laurie Zoloth, professor of medical humanities and bioethics at the Feinberg School of Medicine at Northwestern University, said the United States needs to pay close attention to such work.

"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," she said. "The kind of cloning to make human babies is impermissible. Clearly, the [South Korean researchers'] intent is to do therapy. It's one tiny step closer to some medical use. It would be a wise thing to support."

Additional experiments by the Seoul team suggest its stem-cell colony can indeed generate numerous different types of body cells. It began to form muscle, bone and other tissues in test tubes and when implanted into mice.

The team's next step, now under way, is studying how to direct which tissues those cells form, said Dr. Woo Suk Hwang, lead author of the report.