South Korean scientists say they have succeeded in creating human embryo clones, a major breakthrough for the promising field of stem cell therapy -- and for the far more controversial endeavor of cloning human beings.

The scientists reported that 25% of the egg clones grew to the blastocyst stage. Inside the blastocyst is a small group of cells known as the inner cell mass, which is destined to become the fetus and is the source of stem cells.

Although producing stem cells from clones is a major milestone in the pursuit of stem cell therapy, it is still a time-intensive process.