The advance suggests that someday, a woman who wants a transplant to treat a condition like diabetes or a spinal cord injury could provide eggs to a lab, which in turn could create tissue that her body wouldn't reject.

Ethicists disagreed on whether the strategy would avoid the long-standing ethical objections to creating embryonic stem cells by other means.

The discovery of a mouse embryonic stem cell that is a near-perfect match to human cells will speed research in regenerative medicine and treatments for conditions such as Parkinson's and diabetes, researchers say.

Embryonic stem cells from mice are usually taken from very early stage embryos, called blastocysts (see Instant Expert: Stem Cells). These cells are significantly different from human cells and so have limited value.

Roger Pedersen at Cambridge University, UK, and colleagues took cells from the mouse embryo at a later stage in its development, when it is an epiblast. They found that epiblast stem cells, taken from the innermost layer of week-old rodent embryos, shared many of the same properties of human embryonic stem cells.

It is the third veto of his presidency and the second involving embryonic stem cell research.

Bush said he supports science, but also believes that "all human life is sacred."

Hillary Clinton said today that science, not narrow ideology, should drive our national research agenda, and pledged to lift the ban on ethical embryonic stem cell research. At a conversation at Dartmouth College in Hanover, Clinton was joined by two New Hampshire families with personal experiences with illnesses and disabilities that could potentially be cured or helped by stem cell research, as well as the Associate Chief of Neurology at Dartmouth-Hitchcock Medical Center.

Clinton discussed the potential of stem cell research to both save lives and keep the United States on the cutting-edge of scientific advancement. She argued that ideology should never get in the way of scientific progress.

The first clinical trial of embryonic stem cells is on track to start early next year on patients with spinal cord injury. Geron (NASDAQ:GERN - news), the California-based biotechnology company, will carry out the study on accident victims in six trauma centres across the US.

"The world's spotlight will be on this trial," Tom Okarma, Geron's chief executive, told the Bio conference in Boston. To get it right, the company has carried out several years of preparatory work in collaboration with its academic partners at the University of California, Irvine.

Geron's product will have been tested in 2,000 animals before it goes into its first patient, Mr Okarma said. It consists of immature oligodendrocytes - specialised nerve cells - grown from human embryonic stem cells. The animal tests show that these can repair spinal cord injuries in rats, by growing new nerves with the myelin sheaths they need to work properly. Paralysed rats can walk again.

Diabetes affects 140,000 young Australians, who would die unless they received multiple insulin injections each day of their lives. The cause is the self-destruction of the insulin-producing cells within the pancreas.

Attempts have been made recently to replace these cells with surrogates, allowing recipients to cease injecting insulin. The source of the replacement cells is a pancreas donated after death. But organ supply is limited, with as few as 200 donors in Australia a year.

One gene directs both embryonic and adult stem cells to perform the self-renewal function that is crucial in their potential broad use in medical treatments, researchers said on Thursday.

While the biology of these types of stem cells is very different, a study published in the journal Cell showed that they share at least this one key feature -- a gene called Zfx that controls their ability to self-renew.

Stem cells are a kind of master cell for the body, capable of transforming into various tissue and cell types, offering hope that they can be used to repair tissue damaged by disease or injury.