When President Bush announced the new federal rules for embryonic stem cell research in 2001, it was an attempt to find middle ground between two opposing points of view: that harvesting embryonic stem cells amounted to taking a human life, and that it was an acceptable scientific practice.
Bush's compromise policy was that if scientists had already done the work to create the cells, then federal money could be used to study the results. No federal money could be used to create new cell lines, which would destroy further embryos.
But in the 2 1/2 years since the announcement, the scientific landscape has been changing quickly in ways that have made the federal limits more of a hindrance to science than they first appeared.
When Bush formed his policy, several experiments had hinted that adult stem cells might have the same potential as embryonic stem cells to transform into virtually any tissue in the body. Unlike embryonic stem cells, some kinds of adult stem cells are relatively easy to obtain and are routinely used by doctors, with no ethical concerns. They are found, for example, in the blood and bone marrow.
But more recent experiments have cast grave doubts over the idea that adult stem cells have the flexibility and potential of embryonic ones, undercutting the argument that scientists could rely on adult stem cells for their work.
At the same time, careful scientific reviews of the existing, government-approved cell lines have turned up two other key problems.
Those stem cell lines are kept alive with the help of mouse cells that grow near them, but these mouse cells could also transfer exotic viruses into the cells.
This means that doctors would be loath to use them to treat human patients -- the overriding goal of those in the field.
The cell lines also become less potent with time, according to Dr. Leonard Zon, a researcher at Children's Hospital in Boston and president of the International Society for Stem Cell Research.
By the time scientists devise practical therapies, the approved cell lines may have aged beyond their useful life.
Of the 60-plus cell lines on the government's approved list, about 15 are actually available to researchers, Zon said. Many of the other cell lines initially on the list have exhibited problems -- such as a tendency to change spontaneously -- that make them useless to researchers.
It is possible that more than 15 of the approved lines will be found to be useful, but most scientists doubt there will be significant increases.
This poses a problem for research because each line of cells has unique characteristics, the same way every person does.
A scientist interested in doing a certain experiment could easily go through the available lines and discover that none of them has the characteristics needed to do the research.
Zon said that research has also been hampered because of the complex paperwork that surrounds the cells.
Before a single embryonic stem cell arrives in the lab, scientists must negotiate contracts, called "material transfer agreements," that govern what they can do with the cells, and what stake the original creators of the cells have in any discoveries made with them -- which can delay research by six months or more.