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Scientists have extracted stem cells from embryos in the past, hoping to one day use them as medicine. But they worried that the patient's body would reject stem cells taken from someone else. So for the past several years, various groups around the world have been trying to first create a clone, which would be a patient's exact biological match, then take stem cells from it.

Researchers have said they believe that some stem cells taken from embryos can be coaxed to become almost any type of cell in the human body, a characteristic known as "pluripotency." The stem cells in the Korean study differentiated into retinal, bone and other types of cells when implanted into mice.

Advanced Cell Technology, a private company in Worcester, Massachusetts, announced in a splashy Wired magazine cover story in January that it had cloned an embryo to the 16-cell stage. But the company apparently didn't extract stem cells from the embryo, and it wasn't clear how long the clone survived.

It's significant that the Korean research went through a stringent peer-review process in order to be published in Science, scientists say. Advanced Cell Technology has published just one study in an online scientific journal on its human-cloning experiments. In that study, the researchers -- including Cibelli, who worked there at the time -- achieved a clone with only six cells.