BOSTON, Mass. (November 29, 2007)—No other stem cell is more thoroughly understood than the blood, or hematopoietic, stem cell. These occasional and rare cells, scattered sparingly throughout the marrow and capable of replenishing an entire blood system, have been the driving force behind successful bone marrow transplants for decades. Scientists, for the most part, have seen this as the hematopoietic stem cell’s (HSC) singular role: to remain in the bone marrow indefinitely and to replenish blood and immune system cells only when called upon.