The goal of this project is to perform the pre-clinical and clinical studies that are required for the eventual application of therapeutic gene transfer in sickle cell disease and b thalassemia. Specifically, 1) stem cell mobilization is required for the application of ex vivo stem cell gene therapy but the current approach based on administration of G-CSF cannot be used in sickle cell disease because the resultant leukocytosis precipitates a sickle cell crisis with severe (or lethal) consequences. We will develop new approaches of mobilization that can be used safely in sickle cell disease. Two clinical studies will be done. The first is based on the assumption that recovery from the cytotoxic effects of hydroxyurea results in hemopoietic cell regeneration that is accompanied by increased rate of mobilization of stem/progenitor cells. The second approach is based on the evidence that mobilization using anti-VLA-4 antibody or Flt-3 ligand is characterized primarily by release of progenitor cells and small degree of leukocytosis. Preclinical studies using a combination of anti-VLA-4 with hydroxyurea, or anti-VLA-4 with Flt-3 ligand will be done in baboons to identify a treatment scheme that will result in optimal mobilization with minimal change in white blood cell numbers. The optimal treatment scheme will subsequently be applied to patients with sickle cell disease. 2) We will perform pre-clinical studies aimed to determine whether MLV-based "insulated" bg-globin gene vectors can be used for clinical studies in sickle cell disease and b thalassemia. Specifically, we will a) test whether such vectors can correct the defects in sickle cell and b thalassemia mice; b) assess transduction rates, levels of expression of the transferred bg-globin gene and vector toxicity in the baboon transplantation model; c) determine bg gene expression and transduction rates in primary human erythroid cells following transduction of BFU-E of patients with sickle cell disease or b thalassemia; d) measure transduction rates of human stem cells in the SCID/NOD murine model. 3) Successful completion of these pre-clinical studies will allow us to proceed with a clinical study in patients with Hb S disease or b thalassemia. a) Clinical grade vectors will be produced, b) the necessary regulatory requirements will be fulfilled, c) a clinical study of three patients with sickle cell disease and six with b thalassemia will be initiated to test safety and function of globin gene vectors.