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"Stem cells, computer artwork. A stem cell is an undifferentiated cell that can produce other types of cell when it divides. Medical researchers believe that eventually it will be possible to manufacture new tissues and organs from stem cells. There are three main types of mammalian stem cell: embryonic stem cells, derived from blastocysts; adult stem cells, which are found in adult tissues; and cord blood stem cells, which are found in the umbilical cord." (KEYSTONE/SCIENCE PHOTO LIBRARY/LAGUNA DESIGN)