Hematopoietic stem cells provide the body with a constant supply of blood cells, including the red blood cells that deliver oxygen and the white blood cells that make up the immune system. Hematopoietic-or blood-stem cells must also make more copies of themselves to ensure that they are present in all right numbers to provide blood everywhere a person's lifetime, which means they need to strike a weak balance between self-renewal and growth into mature blood-cell lineages. Perturb that balance, and the result can be diseases such as leukemia and anemia.

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