The renewal of various tissues and organs at steady state or following damage relies upon a small population of locally residing tissue specific "stem cells" Stem cells from adult bone marrow represent an ideal stem cell source based on their ease of collection. In order to begin to explore the potential of adult bone marrow for the correction of genetic diseases that affect the blood such as sickle cell disease, we propose in vitro and in vivo mouse studies to examine the regulation of normal differentiation of hematopoietic stem cells collected from adult volunteers. In order to obtain adult hematopoietic stem cells in large numbers for in vitro and in vivo studies, volunteers will undergo mobilization with G-CSF for 5 consecutive days followed by large volume apheresis on the 5th day of G-CSF injection. The harvested product will be immunomagnetically purified for the primitive progenitor population and viably cryopreserved in multiple aliquots.