Animal cloning is being hailed as one of the century's great scientific breakthroughs. Doctors hope that it will revolutionise medicine, providing an endless supply of organs for transplant and a cure for major killers like heart disease.

Dolly recently had "natural" triplets

But as scientists learn more about the techniques used to manipulate genes and cells, it is becoming clear that cloning may have serious health risks for both the cloned animal and the surrogate mother that is used to bring it into the world.

Many of the clones have developmental abnormalities and die late in pregnancy or soon after being born.

Cloning is clearly possible but scientists do not fully understand how it works

For those that do survive the early stages of life, many of them may be at higher risk of premature ageing and cancer.

Dolly the Sheep, the world's first mammal cloned from an adult cell, appears perfectly normal. But Dr Harry Griffin, from the Roslin Institute, knows there is still much to learn about the technology that created her.

Surprising technology

"In our experiments, we produced 20 live lambs of which six have died soon after birth and a high rate of neonatal death is common in all the experiments to clone both sheep and calves.

"The reason? In cloning from an adult animal you're trying to re-programme a specialised cell, trying to persuade it to start off its life all over again, become a whole animal - it's surprising it works at all."

Meet Mira, the cloned goat

Dolly was cloned from genetic material taken from the udder of an adult sheep. Adult cells, like those in the udder, have become specialised to perform a particular role in the body - in this case milk production. Once cells become adapted to a specific task, they tend to be set in their ways, unlike embryonic cells.

Dr Wolf Reik from the Babraham Institute in eastern England says the secret of cloning is persuading an adult cell to return to its childhood.

"Imagine that you take a cell from a breast cell, in the case of Dolly that was the case," he says.

"Then all the genes that are active to make that a mammary cell, a breast cell, need to be switched off when this nucleus goes back into the egg recipient. Then all the genes that are required for normal development need to be switched on again as development proceeds. That is a very, very complex process."

Increased problems

James Robl from the University of Massachusetts has been looking at a problem that affects cloned calves. The off-spring can become large with an accumulation of fluid in the placenta.

"We know that as you manipulate embryos that the percentage of the calves that have the defect goes up," he says.

The Roslin scientists who created Dolly are having problems cloning pigs

"If we produce an embryo in culture, the rate of the defect goes up to maybe 15%, compared to 1% in the normal population. If we actually produce the embryos through cloning, the rate of the defect goes up to about 50%."

It is likely to be years before cloning can be made safer. And, although the cloning of humans is still in the realms of science fiction, most agree that the potential health risks of cloning must be taken into account in debates on human cloning.

In the publically-funded labs in the US, and throughout Australia, China, Israel, and most European countries, scientists are prevented by law from carrying out research into human cloning.

Human dimension

And many experts agree that although in theory human cloning might be possible, morally it is unacceptable.

"I think there is a general awareness now that this technology is risky, that it would be wholly unsafe to use in humans," says Dr Griffin. "It would be fundamentally wrong to use the technique in young women and I think that there is now more sanity coming into the debate about human cloning.

Allied to other cell technologies, cloning could transform many areas of medicine

"We remain firmly opposed to the copying of individuals, the so-called reproductive cloning."

If scientists really can crack the problems associated with cloning, the potential benefits to humankind are immense. Already animals have been engineered to produce human proteins, such as clotting factors, in their milk. And cloned animals may one day provide organ banks for transplant, tailored to a patient's specific tissue type.