South Korean and U.S. researchers said on Wednesday they had cloned a human embryo and extracted from it sought-after cells called embryonic stem cells.

The experiment, the first published report of cloned human stem cells, means so-called therapeutic cloning is no longer a theory but a reality.

Supporters of medical cloning say it can transform medicine, offering tailored and highly effective treatments for diseases ranging from Parkinson’s to diabetes. They say it could eventually lead to grow-your-own organ transplants.

The stem cells taken from the tiny embryos, known as blastocysts, have the potential to develop into any kind of cell or tissue in the body.

But critics say it involves destroying a human embryo, however tiny, and is thus unethical. The administration of President Bush and supporters in Congress are seeking to outlaw the technology both in the United States and worldwide.

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