FACTBOX: Embryonic stem cell research

(Reuters) - President George W. Bush was set on Wednesday to veto legislation to expand federally funded embryonic stem cell research, aides said.

Following are some facts about stem cells.

* Stem cells are the body's master cells, the source of all cells and tissue, like brain, blood, heart, bones, muscles and skin.

* Embryonic stem cells come from days-old embryos and can produce any type of cell in the body.

* Adult stem cells are harbored in blood and mature tissue in the bodies of children and adults. They are more specialized than embryonic cells and give rise to specific cell types, although they may be coaxed into a broader range of cell types under the right conditions.

* Scientists generally harvest embryonic stem cells from embryos left over after in vitro fertilization attempts at fertility clinics. They can also be produced using cloning technology.

* Scientists hope to harness the transformational qualities of stem cells to provide treatments for a variety of diseases affecting millions of people worldwide, including brain cells for Parkinson's disease, pancreatic cells for diabetes and nerve cells for spinal-cord injuries.

* The issue is controversial because some people believe the destruction of any embryo is wrong. Leaders of the Roman Catholic Church and some other religious and conservative political figures hold this view, though some opponents of abortion rights support embryonic stem-cell research.