Hematopoietic Stem Cells (HSCs) are self-renewing, multi-potential cells mainly residing in bone marrow and are the progenitors of red blood cells, white blood cells and of the cells of the immune system. HSCs can also be isolated from umbilical cord blood and from the peripheral blood. HSCs comprise only a small fraction of bone marrow or umbilical cord blood cells, but are the critical element in HSC transplantation due to their ability to colonize bone marrow and to differentiate into the full complement of hematopoietic cells.