Donors: Stem cells are collected from the donor. To do this, the hormone granulocyte colony stimulating factor (G-CSF) is injected under the skin for several days to move stem cells out of the bone marrow into the bloodstream. Then, the cells are collected by apheresis. In this procedure the blood is drawn through a needle placed in one arm and pumped into a machine where the stem cells are separated out and removed. The rest of the blood is returned to the donor through a needle in the other arm.