Hematopoietic stem cells (HSC) provide for contmuous replenishment of the entire immune and hematopoietic systems, and also replenish themselves in a process termed self-renewal (1).The HSCs can be enriched from hematopoietic tissues using MAbs that bind to the CD34 antigen, a universally recognized marker for hematopoietic progenitors (2-4).Enriched HSC populations are being widely investigated for use in transplantation and gene therapy because they appear to provide rapid hematopoietic reconstitution in myeloablated patients (5-11), and they offer good targets for gene transfer (12-17).