Administration of G-CSF is essential to allow collection of sufficient numbers of primitive stem and progenitor cells from normal volunteers. In the dose to be used in this study, a 15-35 fold increase in the concentration of CD34+ cells occurs in peripheral blood over the 4 days of G-CSF administration resulting in an apheresis product which contains, on average, about 1-2 x1010 nucleated cells from which 1-2x108 purified CD34+cells can be recovered. This number is sufficient to allow an experimental design which includes in vitro culture, transduction with vector particles and assay of transduced cells in the immunodeficient mouse model.