Reaction is mixed in the international scientific community to the first successful cloning, or copying, of a human embryo by South Korean scientists. Researchers who support the work say human embryo cloning could potentially go a long way toward treating, or curing, devastating illnesses, such as Parkinson's disease and diabetes. But opponents say the copying of human embryos is unethical and amounts to abortion. Research will continue as both sides debate the issue.

Some scientists in the field of biotechnology say they will build on the work of South Korean researchers, who are the first to report the successful cloning of human embryos. But the development creates a number of dilemmas, according to Laurie Zoloth, a bioethicist at Northwestern University in Chicago, Illinois.

On the one hand, Professor Zoloth says, the creation of human embryos by researchers at Seoul National University will inflame the passions of those vehemently opposed to any research involving human eggs, because they think it amounts to abortion.

On the other, Professor Zoloth says the work holds great promise.

"With more research and much more careful study, we just might be heading toward fundamental changes in how we treat human health," she said.

So-called human reproductive cloning is nearly universally opposed by those both in and out of the scientific community, because some people might want to create exact genetic copies of themselves.

But so-called therapeutic cloning is supported by those who believe it has the potential to cure human disease. The process involves creating an embryo by fertilizing human egg cells with the genetic material of other cells and harvesting the stem cells.

Embryos are a rich source of stem cells, master cells in the body which can, in theory, be genetically coaxed into transforming into any cell in the body. Experts say cloning would offer an unlimited supply of stem cells that could potentially be used to repair or replace cells damaged by disease.

Rudolph Jaenisch is a researcher at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts.

"I do support therapeutic cloning," said Rudolph Jaenisch. "I think it is potentially a very important therapeutic approach to help patients with devastating diseases, like Parkinson's and diabetes."

The Bush administration opposes research using human eggs harvested after 1981, and those eggs must come from existing stocks around the country.

South Korean researchers were motivated by the fact that stem cells generated from existing embryos would be rejected by the bodies of the recipients.

"Since [cloned] embryonic stem cells are derived from the patient, him or herself, there's no need for immune suppressive therapy, which you would normally use, if you would make a transplantation from some other embryonic stem cell, which is not related to the patient," said Whitehead Institute's Rudolph Jaenisch.

The research describing the cloning of human embryos by South Korean researchers was published in the journal Science.

In announcing the development, Science editor-in-chief Donald Kennedy said it could be a long time before anyone knows whether therapeutic cloning would benefit humans.

"It does offer promise, but I emphasize it is long-range, rather than short-range, promise," he said.

In the meantime, Northwestern University ethicist Laurie Zoloth predicts issues concerning human embryo cloning will be widely debated.

"And many of these, the duty to heal the sick and the need for free scientific inquiry will be the primary consideration," said Laurie Zoloth. "For many whose religions prohibit any use of the human embryo, no matter how it is created, much of this research will remain impermissible."