The ban does not affect research into stem cells taken from children or adults -- known as "adult stem cells" -- or cells taken from embryos that were already undergoing research by that date. And it does not affect the cells that are regenerated from this group of embryos.
But in an address broadcast to the nation from his Texas ranch when he imposed the restrictions, Bush -- who is deeply religious and a strong opponent of abortion -- said he could not let the U.S. government continue backing research on additional embryos because of the ethical questions it raises.
"This issue forces us to confront fundamental questions about the beginnings of life and the ends of science,” Bush said. “It lies at a difficult moral intersection, juxtaposing the need to protect life in all its phases with the prospect of saving and improving life in all its stages."
Stem cells are cells that are immature but destined to develop into specialized cells, such as those in the muscles, blood, or brain. They must be extracted from human embryos. Most often, U.S. scientists use frozen embryos from fertility clinics that are no longer wanted by the couples that had them created.
Obtaining stem cells from embryos destroys the embryo. Many religious groups and other stem-cell opponents believe that human life begins at conception, and that the destruction of an embryo therefore represents murder, much as abortion does.
But stem-cell proponents say there are many benefits to such research. Dr. Timothy McCaffrey, a medical researcher at George Washington University in Washington, told RFE/RL that he expects stem cells will one day provide valuable treatment for diseases such as Alzheimer's and Parkinson's diseases, diabetes, and even heart disease.
"A variety of diseases, maybe almost all diseases, to some degree involve the degeneration of tissues, and that usually means the death of cells within them. Stems cells are really about a new field called 'regenerative medicine,' whereby once damage is done to a tissue, there might be the ability to turn stems cells into the particular type of cell that you want and allow that organ to go back to functioning the way it's supposed to," McCaffrey said.
McCaffrey said that embryonic stem cells are so far known to be more versatile than adult stem cells because they are believed to be able to grow into any kind of organ tissue. He said adult stem cells, on the other hand, do not seem as versatile. However, McCaffrey said he is involved in research that may allow a stem cell from the pancreas, for example, to assume the functions of another organ, such as the heart.
McCaffrey said the benefits of stem-cell technology are potentially unlimited. In importance, he ranks it with genetic research as a prime force in modern medicine.
But the limits imposed by Bush may shift the most serious research into stem cells out of the United States, according to Ronald Bailey of the Competitive Enterprise Institute, a private policy-research center in Washington. Bailey told RFE/RL that some prominent U.S. researchers in the field have moved out of the country to pursue their studies in what they consider less restrictive environments. He says this could become a trend and hurt the United States' prestige as a center of leading medical studies.
"The problem with putting restrictions on stem-cell research in the United States is that the United States could do it better, faster, and more ethically, probably, than most other countries. We have a very strong oversight system for science in this country, for ethics. And the problem is that other places -- like South Korea, perhaps, and China most particularly -- will be forging ahead with this research. And they don't have that same kind of ethical infrastructure and legal infrastructure that we have," Bailey said.
Already, Bailey said, other countries have taken the lead in stem-cell research. He said the United States now must watch others lead the way.