Most cloning is carried out by a process known as nuclear transfer, where the nucleus is removed from an egg and replaced with the nucleus of the animal to be cloned. Here, the nucleus is removed from a cell.
South Korean scientists announce the world's first successfully cloned human embryo. Unlike other past cloning claims, the scientists have reported their work in a prestigious, peer-reviewed journal, Science. Researchers say the breakthrough could lead to an entirely new kind of medical treatment.
The scientists with the Seoul National University were not trying to clone a human. Instead, they were searching for ways to make embryonic stem cells, a type of cell that can turn into any cell type in the body. But to make embryonic stem cells, researchers need an embryo.
The basic process, called nuclear transfer, for making a cloned human embryo is the same as for other animals. Genetic material is removed from a human egg and replaced with the genetic material from an adult cell. In the Korean experiment, both the egg and adult cell came from the same woman. The embryos were allowed to grow for several days -- to the blastocyst stage -- at which point the researchers attempted to harvest stem cells from them.
It took 242 eggs to get a single culture of embryonic stem cells to grow. The Korean scientists say they do not plan to conduct further cloning experiments, but will instead focus on developing therapies from the stem cells they've produced.
As NPR's Joe Palca reports, the news has reopened the contentious debate over cloning. Scientists say cloning offers a unique way to produce cells that may someday be used to treat diseases. But critics argue that any form of cloning is morally repugnant, and should be banned.