By Helen Briggs
BBC News science reporter
US scientists say they have produced embryos that are clones of two men, in an attempt to produce patient-specific stem cells.
Stem cells are the 'master cells' of the body
Researchers removed DNA from donated human eggs, and replaced it with DNA from the skin cells of two volunteers.
They produced embryos with genetic material that matched the men's, but did not go on to extract stem cells.
UK experts say the research, published in the journal Stem Cells, is a small but not a great step forward.
The team at Stemagen Corporation in La Jolla, California, says the work could be an important stage in developing embryonic stem cells for patients.
The group produced five embryos called blastocysts from 25 donated eggs. DNA fingerprinting proved that at least one of these was a clone.
"We're the first in the world to take adult human cells and then document that in fact we were able to clone embryos from them," lead researcher Dr Samuel Wood told the BBC.
He said the embryos were destroyed in the process of verifying they were clones, but they were now working on creating stem cell lines.
Dr Lyle Armstrong of Newcastle University is one of a handful of other researchers who have made cloned human embryos using a technique known as nuclear transfer pioneered in Dolly the sheep. Unlike the US team, the Newcastle group used DNA from embryonic rather than mature tissue.
Dr Armstrong said the US study showed that the objective of using cells from an adult person to make individual stem cells might one day be possible.
"It's a small step but not a great step forward," he told BBC News. "It's interesting that they've been able to repeat somatic cell nuclear transfer and get embryos of the stage where embryonic stem cells could be derived, but it is disappointing that they've failed to derive a stem cell line."
Many scientists believe that being able to make stem cell lines tailored to individual patients could revolutionise the treatment and prevention of human diseases.
But the research has proved controversial. Korean scientist Hwang Woo-suk claimed in 2005 that he had created such cell lines, but the study was later discredited. Meanwhile, critics have objected on ethical grounds, saying it is wrong to use embryos for research.
Some scientists argue that clones might not be required to harvest stem cells. Last year, researchers in Japan and the US were able to "rewind" adult cells back to their embryonic state using a new technique.
Professor Jack Price of King's College, London, is an expert on neural stem cells. He too said the Californian experiment was a small step forward but not a breakthrough.
"This constitutes technical progress," he said. "It shows that the approach using human embryos does still have promise and it does provide justification for continuing that avenue of research."
TECHNIQUES FOR MAKING 'STEM CELLS'
Therapeutic cloning produces stem cells which can develop into different types of body cell, making them ideal for research into treatment of disease.
But this technology involves the creation and destruction of embryos, which is ethically controversial. The stem cells created also run the risk of being rejected by the body.
The new technology, nuclear reprogramming, creates stem-like cells from the patient's own cells, avoiding both these problems.