California scientists recently took a major step toward creating a cloned human being. Is it only a matter of time until human clones walk the earth?

How close are scientists to cloning humans?
They’re essentially one step away. Last month, a La Jolla, Calif.–based biotechnology firm, Stemagen, announced it had created the first cloned embryos from adult cells—a major breakthrough in the science of cloning. Skin cells donated by two adult men were implanted in 29 human eggs that had been hollowed out and stripped of their DNA. Five of the resulting embryos survived for several days—long enough to reach a crucial developmental stage known as a blastocyst. Each young embryo was an exact genetic replica of one of the men who had contributed the skin cells. If these embryos were implanted in a woman’s ovary and allowed to mature, they would have come into the world as genetic carbon copies of their fathers. Soon, scientists will try to build on this work by harvesting stem cells from cloned embryos. “We are waiting with bated breath,’’ said Stephen Minger, a stem-cell biologist at King’s College, London.

Why all the excitement?
Because it would mean that science could take a cell from any adult, and transform it into genetically identical, healthy new cells or organs. These tissues and organs could then be implanted into people suffering from such degenerative diseases as Alzheimer’s, Parkinson’s, or multiple sclerosis. Numerous technical hurdles must be overcome before this so-called therapeutic cloning can be realized. Still, said Dr. Samuel Wood, Stemagen’s chief executive, “We’ve at least shown the opening to the cave that is the Holy Grail.” But some view such a prospect with horror.

What are the critics worried about?
They fear that the cloning process could be taken well beyond stem-cell extraction. Theoretically, cloned human embryos could be implanted in a womb and brought to full term. Scientists have peformed such “reproductive cloning’’ with animals since 1996, when Scottish scientists made world headlines by cloning a sheep they called Dolly. If they chose, the Stemagen researchers could have tried to create full-fledged human beings from their five cloned embryos. The scientists said they destroyed the embryos because they had no interest in trying to grow them into actual people. “It’s unethical,” said Wood, “and we hope no one else does it either.”

What if somebody does?
It would create a moral quandary that until now has been the stuff of science fiction. It’s not hard to imagine how reproductive cloning could be perverted or abused. Critics fear that unscrupulous scientists might try to use cloning to practice eugenics, breeding humans with blonde hair, athletic prowess, high IQs, or whatever characteristics they deem worthwhile. The dilemmas of reproductive cloning, says Stanford professor William Hurlbut, a member of President Bush’s Council on Bioethics, “get to the core of what it means to be a human being.” Indeed, the very process of attempting to clone a human raises wrenching moral issues, because it is so dangerous and unpredictable.

How dangerous is it?
For every cloning success, there have been many more failures. In the decade since Dolly was born, scientists have cloned many healthy goats, cows, mice, pigs, cattle, and rabbits—healthy enough for the FDA last month to deem them safe for food purposes. But despite these advances, most clones are stillborn or die soon after birth. Hundreds of eggs may be needed to yield a single mature specimen. Dolly herself was born only after 277 attempts. Even when a clone grows to adulthood, there is no guarantee it will be normal. Indeed, about 30 percent of cloned mammals are affected with a form of gigantism called “large offspring syndrome” or with other debilitating conditions. Some of these problems may not become apparent for years.

But is it technically feasible to clone a human?
That’s still unclear. It has become apparent that certain species simply can’t handle the process of cloning as well as others. Attempts to clone horses, dogs, and chickens, for instance, have all failed. It wasn’t until last November that the first cloned monkey embryos were produced, and some scientists suspect that primates are particularly clone-resistant. “For each species there’s been a unique set of problems,” said biotechnology researcher Robert Lanza, “and the human is no different.”

Are there restrictions on human cloning?
Yes, but they are weak. In 1998, 19 European nations signed a protocol to ban any attempt “to create human beings genetically identical to another human being.” But three years later, the European Union voted overwhelmingly against such a ban, out of concern that it would inhibit promising medical research. In 2005, after four years of debate, the U.N. General Assembly adopted a resolution calling on all governments to ban human cloning as “incompatible with human dignity and the protection of human life.” But that resolution is nonbinding. In the U.S., the House of Representatives has voted to ban human cloning, but the Senate has yet to follow suit.

So essentially, human cloning is legal?

As of now, yes. In general, most governments favor therapeutic cloning and oppose reproductive cloning. In November, the U.N. University Institute of Advanced Studies urged the nations of the world to pursue therapeutic cloning but to eschew reproductive cloning because of the moral implications. But no one has yet come up with a hard and fast line that divides one from the other. One thing is sure, said Brendan Tobin, a human-rights lawyer who co-authored the U.N. report: “Failure to outlaw reproductive cloning means it’s just a matter of time until cloned individuals share the planet.”