Christopher Reeve, the actor who portrayed Superman in four Hollywood movies, died Sunday. He had been in a wheelchair since a 1995 horseback riding accident left him paralyzed from the neck down. For the past nine years, he was a tireless advocate for research to cure paralysis.

Mr. Reeve believed, as do many scientists, that stem cell research could help provide the answer to those with spinal cord injuries.

Stem cells have the unique ability to develop into a variety of cell types. Embryonic stem cells, as the name suggests, are present at the earliest stage of life. There are also adult stem cells.

As scientists learn more about stem cells and how they might help those with spinal cord injuries, Alzheimer's Disease, stroke, and other conditions, stem cell research has emerged as a political issue. Here's an excerpt from last week's presidential debate. First, the Democrat, Senator John Kerry.

"And I believe if we have the option, which scientists tell us we do, of curing Parkinson's, curing diabetes, curing some kind of a paraplegic or quadriplegic or a spinal cord injury, anything - that's the nature of the human spirit," said Mr. Kerry. "I think it is respecting life to reach for that cure. I think it is respecting life to do it in an ethical way.  And the president's chosen a policy that makes it impossible for our scientists to do that.

In reply, President George W. Bush referred to ethical issues surrounding the use of human embryonic cells.

"But I think - I think we've got to be very careful in balancing the ethics and the science. And so I made the decision we wouldn't spend any more money beyond the 70 lines, 22 of which are now in action, because science is important, but so is ethics. So is balancing life. To destroy life to save life is one of the real ethical dilemmas that we face."

Opponents of embryonic stem cell research say the source of the cells raises ethical problems, because they believe that an embryo has the same rights as a person.

Federal funding for embryonic stem cell research is limited to certain cell lines; much more funding is allotted to adult stem cells. Laboratory studies of adult stem cells have shown impressive results, according to David Prentiss of the Family Research Council, which supports traditional values.

"The trick has been, how do we turn them up so we can accomplish these sorts of more radical treatments with heart disease, with stroke, with Parkinson's. But again, the adult stem cells have already been showing that success - not just in the animals, but in the patients as well," he said.

Dr. Prentiss says he does not believe that embryonic stem cells have any unique qualities.  But we get a different view from Dr. Robert Lanza of Advanced Cell Technology, a Massachusetts biotech company.  He says the value of adult stem cell research is real, but limited. "They are very useful. The question here really is, will they be useful for all diseases? I think the consensus in the medical community is that they don't have the same versatility as embryonic stem cells," he said.

Most American voters probably will not choose between George Bush and John Kerry based on their positions on stem cell research. In California, however, voters have a clearer choice. A ballot measure known as Proposition 71 would authorize the state to borrow $3 billion to finance stem cell research. Three weeks before the November 2 vote, polls indicate voters are likely to narrowly approve the measure.

Groups such as Cures for California support the measure; opponents include groups such as Doctors, Patients & Taxpayers for Fiscal Responsibility.