The same technology—cloning—that could one day cure diseases like Parkinson's, diabetes and even heart disease, could also be used to make a copy of a human being.

This ScienCentral news story looks at the promise of human cloning, and the danger of cloning humans.

Human Cloning vs. Cloning Humans

When scientists say "cloning," they mean somatic cell nuclear transfer. To make a clone, scientists remove the nucleus from an unfertilized egg, and replace it with a nucleus from an adult body cell such as a skin cell. In doing so, the resulting cell somehow gets "reprogrammed"—turning off the genes that made it behave like a skin cell, while turning on genes that make it act like a developing embryo. The "pre-embryo" is grown in the lab for a day or two, to just about 100 cells.

What happens next is what separates human cloning from cloning humans. In "reproductive cloning," the pre-embryo is implanted into a female's uterus, where it may develop into an exact copy of the adult animal that donated its body cell DNA—like Dolly the sheep. In so-called "therapeutic cloning," its development is stopped and scientists can culture from it millions of embryonic stem (ES) cells. Researchers are quickly learning how to coax ES cells into differentiating into all sorts of body cells that could be used to replace cells and tissues damaged by disease or aging.

Rudolf Jaenisch, a leader in nuclear transfer research, has become an advocate against attempting to clone humans. Research at his MIT lab raises big concerns about the safety of reproductive cloning. Their latest study, published in Proceedings of the National Academy of Sciences, used high tech gene chips to analyze whether normal-looking mouse clones have genes that function normally.

"A year ago we only guessed, we only thought, they probably are not normal," Jaenisch says. "Now we know for sure, they are not normal."

Jaenisch says that makes it simple to oppose reproductive cloning, at least for the foreseeable future. "From a scientific point of view, we know with a high degree of certainty that any baby one would derive through this nuclear transfer technology—well, most of them would die [before birth]. But the few which make it to birth will be abnormal. Some would be dying very early, some may develop to let's say school age, but they will definitely be abnormal," he says.

The majority of scientists agree. In January, the National Academy of Sciences released a report on the scientific and medical aspects of human reproductive cloning that concluded it would be so dangerous, the United States ought to ban it.

No therapies... yet

Therapeutic cloning hasn't yielded any new therapies yet. But scientists are so convinced it someday will, their pleas persuaded President Bush to permit limited federal funding of ES cell research for the first time in US history (see "Human Cloning - The Ethics").

The first proof that therapeutic cloning can work in an animal model of human disease came this year, when two MIT labs collaborated to create a mouse embryonic stem cell line from a body cell of a mouse with an immune deficiency. "This was a mouse model of the bubble boy disease," says George Q. Daley whose lab collaborated with the Jaenisch lab on the research.

"Because that line was derived from the original mouse, it was genetically identical to it, and so it carried the same genetic lesions that produced the immune deficiency," says Daley. "In the petri dish, we were able to correct those genetic defects... And then using methods we've just recently developed, were able to direct the development of blood stem cells from these ES cells, and put them back into the original mouse."

The therapy partially restored the mouse's immunity. "As with many firsts, it was not 100 percent effective," Daley says. "But, it at least convinced us that with the technology available today, we could put this into practice."

As enlightening as it might be to create a new human ES cell line from a child suffering from the disease, the scientists can't undertake this under the President’s funding policy. The same huge majority of scientists that favors banning reproductive cloning wishes the United States would support therapeutic cloning.

But as Daley says, "it would require a change in the presidential policy for the Federal government to support this work."