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Adult stem cells. Fluorescence light micrograph of adult stem cells identified by a laser cytometer scanner. The cell nuclei are yellow. Stem cells are cells that differentiate to form the range of different tissues found in the human body. The most potent stem cells are found in embryos. Other types of stem cells are found in adults. They are less potent than embryonic stem cells, only being able to mature into a specific type of tissue, such as bone marrow stem cells maturing into blood cells. It is thought the brain, and some other tissues, also contains adult stem cells. Stem cells are being studied as treatments for a wide range of disorders. Photographed in France. (KEYSTONE/SCIENCE PHOTO LIBRARY/PHILIPPE PLAILLY)