Additional Study Information: We are doing a research study to find out whether it is safe to use a virus to change the genes of bone marrow cells that become red blood cells. We hope that this type of therapy may be helpful in people with sickle cell disease, but it has not been tested before. Genes, which we inherit from our parents, are pieces of information which provide the blueprints for all the proteins that keep our cells and ourselves alive. Sickle cell disease is caused by a beta globin gene that does not behave in a normal and expected way. This is due to a change in the gene (mutation) that leads to abnormal red blood cells, in turn leading to anemia, pain and possible damage to your internal organs. This study is evaluating an experimental procedure for treating sickle cell disease called: gene transfer. Experimental means that this procedure has not been approved by the U.S. Food and Drug Administration (FDA), but is being tested to find out if it is safe and if it works. In this study, the gene that is being transferred into the cells is a modified version of the beta globin gene that is mutated in the red blood cells causing sickle cell disease. It is hoped that by inserting a modified beta globin gene without the sickle cell mutation into the stem cells, the symptoms of the sickle cell disease will be diminished or eliminated. This result has been seen in studies of mice with a blood disease similar to sickle cell disease. In these mice, gene transfer with a modified version of the beta globin gene successfully treated the blood disease. Based on these data, this research study was opened in the hope that the functioning beta globin gene (without the sickle cell mutation) will be able to correct the abnormalities causing sickle cell disease disease. It is unknown if this will happen in human subjects also.