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Stem cells can be derived from various sources, including adult tissues (such as bone marrow and adipose tissue), embryos, and umbilical cord blood. Adult stem cells, also known as tissue-specific stem cells, are found in specific tissues and are generally difficult to grow and expand in the laboratory. Hematopoietic or blood-forming stem cells in the bone marrow, for example, only make up one in a hundred thousand cells of the bone marrow but can be isolated and used in treatments like bone marrow transplants.