Candidates take opposing stances on medical research.
Before President George W. Bush arrived in the Oval Office, most Americans had never heard of a stem cell - a microscopic biological entity that can transform into hair, muscle or other human cell types. But four years on, the issue has escalated into a divisive one in US politics, and looks set to attract continued attention in the forthcoming election.
Stem cells started invading the public consciousness on 9 August 2001, when Bush appeared on national television to declare a new policy on stem-cell research. He stated that scientists funded by the federal government could not derive fresh stem cells from human embryos because, he said, it is immoral to destroy embryos for the purposes of human research. Researchers could work on embryonic stem-cell colonies that were already growing in culture dishes - and were free to study adult stem cells, which are harvested from more fully developed tissues and grow into a more limited range of cells.
Scientists have been frustrated by this rule for three years. They say that it is slowing progress in stem-cell research, because the number of existing embryonic stem-cell lines is too few and there are burdensome rules surrounding their use. This hampers their ability to pursue stem-cell therapies that, they say, might one day cure conditions such as diabetes or heart disease.
The scientists have gained an audience and a spotlight. In May, Nancy Reagan announced that she supports embryonic stem-cell research to cure Alzheimer's disease, which claimed the life of her husband, former President Ronald Reagan. Also this year, bipartisan groups of Congressmen and of Senators each wrote to Bush to ask him to change his policy. Some states, such as California and New Jersey, have passed their own measures to support human embryonic stem-cell research.
In his replies to Nature's questionnaire on science issues, John Kerry says that he will lift the restrictions Bush has placed on stem-cell research while ensuring ethical lines are not crossed. Most stem-cell biologists support this plan.
But scientists caution that, if elected, Kerry must work carefully with politicians and scientists to hammer out a stem-cell policy that frees up research while ensuring that the collection and manipulation of embryos to make stem cells is properly regulated. "My hope is that whoever takes office will see the light and work with Congress to give us a policy that makes sense," says Lawrence Goldstein, a cell biologist at the University of San Diego, California, who supports embryonic stem-cell research.