In the hematopoietic system, the properties of differentiation,multipotentiality, and self-renewal were first demonstrated more than 40 years ago through a series of experiments showing the ability of a subset of cells within the bone marrow (BM) to form colonies when transplantated into lethally irradiated recipient animals (Till & McCulloch, 1961). Hematopoietic stem cells (HSCs) were the first tissue-specific stem cells to be isolated and are the main stem cells in routine clinical use to date in the treatment of a variety of blood cell diseases such as leukemias and autoimmune disorders.