Hematopoietic stem cells in the bloodstream are blood-forming stem cells that continuously circulate in the body in small amounts. However, when the body receives cytokines that stimulate bone marrow activity, such as G-CSF (a naturally occurring factor that stimulates stem cell production), or during bone marrow recovery after chemotherapy, the number of these blood-forming stem cells in the bloodstream increases rapidly. Transplanting stem cells collected from the bloodstream tends to engraft more quickly than using stem cells from bone marrow, possibly because they contain more mature blood-forming stem cells. When we refer to ‘stem cell transplants,’ it usually means the transplantation of stem cells from the bloodstream. Although the natural habitat of stem cells is the bone marrow, we can induce them to exit the bone marrow and enter the bloodstream, where they can be collected more easily.