But all this was still just science fiction until 1996, when Scottish scientist Dr. Ian Wilmut and his team successfully cloned a sheep and named her Dolly. Since then, as many as 5,000 animal clones have been created in laboratories around the world. Then, in November, scientists at Advanced Cell Technology, a company in Massachusetts, announced a dramatic new breakthrough: They claimed to have cloned the first human embryos.

What Is a Clone?
A clone is any organism that's formed from exactly the same DNA as another. When an animal is created through normal channels, it gets half its genetic material from its mother, and the other half from its father. But when a clone is created, it shares all of its DNA with the animal it's cloned from. This makes it an exact genetic copy of the first animal: they will look identical, because they have all the same DNA.

Although clones may seem to be the stuff of science fiction, there are actually clones that happen naturally all the time; they're called identical twins. Identical twins come from one fertilized egg that splits, creating two identical-looking people.

Of course, creating clones in a lab is a whole different thing. And when Dolly was born, there was a lot of controversy. Most of it centered around the idea that we were taking the first steps toward cloning humans. Now that human embryos have been cloned, the debate has started all over again.

The Two Faces of Cloning
When people talk about human cloning, they generally talk about doing it for two different reasons. The first is to create new babies--what's known as "reproductive cloning." The second is to clone cells that would never grow into real people, but could be used to treat diseases. This is known as "therapeutic cloning."

This is what the scientists at Advanced Cell Technology are working toward. Their ultimate aim is to create cloned embryos of adults, and take the stem cells from those embryos. Those stem cells could, in theory, be used to replace damaged or diseased cells, and cure a whole range of diseases and injuries. The debate over stem cell research has been raging for a while. But many scientists think that cloning might be the best way to gather stem cells. If stem cells come from another source, the patient's body could reject them. But a person's cloned embryo would contain that person's exact DNA. Cells and even organs created from those stem cells would be, in theory, exactly like the ones from the person's own body. There would be little chance of the patient's body rejecting them and the risk of complications would be much lower.

But the debate over therapeutic cloning rages on. Over the summer, the U.S. House of Representatives passed a bill that would ban all human cloning, including therapeutic cloning. But the Senate has not acted on this bill, and there are many who argue that we should not stop this research.

Let's take a look at both sides:

Side One: "Yes! Send in the Clones!"
There is a small but passionate group of people who say we should be able to make babies through cloning. One of them, Dr. Brigitte Boisselier, is the director of a company called Clonaid, which is working to clone a human being. Clonaid made headlines recently when they announced that they were attempting to clone a small child who had died tragically. Dr. Boisselier argues that infertile couples and single people should be able to have the option to clone themselves if they want to have children. She points out that when the first "test tube" baby was born, many people said she would be somehow damaged or that she'd be treated differently. Now in-vitro fertilization is common. "Why should we be afraid of a baby who is born and raised and loved," says Dr. Boisselier, "even if it comes from a different environment?"

And many scientists, ethicists, lawmakers, and others who think the idea of making babies through cloning is immoral or disturbing feel we should clone embryos for medical science. Those in favor of therapeutic cloning say that the medical benefits of this technology are too great to ignore, and it would be wrong to stop it. Stem cell therapies could in theory cure injuries and diseases ranging from spinal cord injuries to heart problems to leukemia, Parkinson's disease, diabetes, and Alzheimer's. These are illnesses and problems that affect millions of people every year. Michael West, the president of ACT, where the embryos were cloned, says that stopping this research would keep 3,000 people a day from getting help.

Supporters say that the cloned embryos that are created are not viable children, or even fetuses. As Rep. Jerrold Nadler of New York argues, "We must not say to millions of sick or injured human beings, 'go ahead and die, stay paralyzed, because we believe the…clump of cells is more important than you are.'"

And many argue that banning human cloning will only drive this research underground or out of the United States to places like Great Britain, where therapeutic cloning is allowed. This could slow the progress of the research, and might even mean that treatments and technologies available in other countries would not be available in America.

And many argue that in order to advance medical science, we have to take these kinds of steps. "There is lots of risk in life," says Dr. Judson Somerville, a paralyzed doctor who contributed some of his own skin cells for the research. "And sometimes in order to advance science and to make this a better world you have to take some risks."

The Flip Side: "No! Stop Cloning Around!" Those who argue against cloning feel it's like playing God. Embryos would be created specifically for their stem cells, and once the stem cells were taken, those embyros would not survive. Opponents who believe that life begins at the moment of conception say that it is wrong to create life only to destroy it, no matter what the reason. This was President Bush's reaction to the latest cloning announcement. "The use of embryos to clone is wrong," he said. "We should not as a society grow life to destroy it."

Some opponents say that we are opening the door to a world of "embryo farms," where clones are created to grow organs and body parts. Doing this, they argue, turns cloned embryos into tools, or commodities.

And many argue against the idea of reproductive cloning. It strikes many as morally wrong or unnatural to create and raised a cloned person. Others fear that it opens the door for more tinkering with birth and contraception. On the one hand, medical science could come up with ways to eliminate diseases, like cystic fibrosis, that babies are born with. On the other, will it lead to a world full of "designer" babies with super-genes, all created in a laboratory?

And some worry about the potential for problems and defects in cloned people. Dr. Wilmut, the man who led the team to clone Dolly, had to try 277 times before he successfully cloned a sheep who survived and appeared to be free of defects. Although many cloned animals appear to be healthy, it is still unknown if they will age faster than "normal" animals, or suffer other complications. How many "bad copies"--birth defects, mutations, etc.--are we willing to tolerate to get cloning right? For his part, Dr. Wilmut has spoken out against human reproductive cloning, calling the chance of failure "unacceptable."

What Do You Think?
Should we clone embryos to cure diseases? How do you feel about people cloning their dead children, or raising clones of themselves? Weigh in with your opinion!