Stem cells are the master cells found in embryos that give rise to all other cells in the body. Doctors hope they will be able to cure or treat hundreds of diseases by directing stem cells to develop into any type of tissue needed for transplant. Extracting stem cells involves destroying the embryo. Meanwhile, U.S. President Bush said on Friday that federal money should not pay for research on fetal tissue or stem cells derived from abortions. Bush had indicated his opposition to such research during the presidential campaign, but the remarks Friday were his first on the topic since taking over the White House a week ago.
Stem cells can come from aborted fetuses or from embryos left over from fertility clinics.
A few also can be found from adult tissues, but scientists say fetal and embryonic stem cells are the most flexible and thus most usable. Bush did not specifically address embryonic stem cells. The Associated Press contributed to this report. Pages will open in a new browser window External sites are not endorsed by CNN Interactive.