Stem cells, which the bone marrow produces, are responsible for making all the different kinds of blood cells. They are the cells used in bone marrow, or stem cell, transplantation. The drug G-CSF, which is a naturally occurring hormone, causes stem cells to mobilize-that is, to be released from the bone marrow and enter the blood stream. This drug is given to stem cell donors to increase the amount of cells that can be collected. Stem cell donors for patients with sickle cell disease are often healthy siblings of the patient who have a matching bone marrow type. Some siblings carry the sickle cell trait, however, and, even though they do not have sickle cell disease and their blood and bone marrow are normal, it is not known how their cells will react to G-CSF stimulation. Nor is it known if their stem cells require special methods of removal, processing or storing.