It was an electric moment--the declaration of a milestone--couched in the precise language of science. "I am happy to announce the successful derivation of human embryonic stem cells from cloned human blastocysts," Dr. Woo Suk Hwang told a packed audience in Seattle last week. Hwang, of Seoul National University, and his team harvested 242 eggs from 16 female donors, added the DNA of adult cells and developed 30 cloned embryos. One embryo produced stem cells--the prized blank slates that scientists believe can be coaxed into brain, muscle and other cells so that one day illnesses like Parkinson's can be cured and hearts can be patched as easily as bicycle tires. The achievement, says Dr. Robert Lanza of Advanced Cell Technology, a U.S. firm working toward similar goals, "could help spur a medical revolution as important as antibiotics and vaccines."

The experiment, published in the journal Science, quickly accentuated the divide between those who believe cloning should be outlawed entirely and those, including most scientists, who support banning it for reproductive purposes but not for medical research. Hwang emphasized that he undertook his work solely to advance therapeutic cloning. Proponents say the procedure is markedly different from making carbon copies of human beings. "There is no implantation; there is no pregnancy," says Daniel Perry, head of the Coalition for the Advancement of Medical Research. "This is a technique to develop tailor-made stem cells for patients in a glass dish." But the creation of human embryos for research and the reality that researchers now have a recipe that could lead to human clones sparked outrage. Opponents like New Jersey Rep. Chris Smith, chair of the bipartisan Congressional Pro-Life Caucus, called the work "sick in the extreme."

There are no federal laws against cloning, but steps have been taken to curtail experimentation in the United States. In 2001, President George W. Bush restricted federally funded stem-cell research. The House has twice passed legislation that would ban all human cloning. A similar bill, as well as a competing one to outlaw only reproductive cloning, is pending in the Senate. Many scientists worry that the United States will become a bystander to medical innovation that could save millions. Their opponents say last week's news makes banning research on human embryos more imperative. Both sides agree on one thing: lives are at stake.