Our objective is to establish an effective protocol for enrichment or purification of blood stem cells, using a one-step procedure. We plan to achieve this by means of a stem cell-specific monoclonal antibody. Using a polyclonal antibody, we have shown in our recent publication that a molecule, gp105, is present specifically on blood stem cells. In addition, we presented data in this application that single gp105-sorted cells could be used to repopulate the blood system in lethally irradiated recipients. in this Phase I application, we propose to screen for a monoclonal antibody specifically directed against gp105. Once obtained, we shall verify that it can be used to show that its expression is restricted to blood stem cells, and that it can be used to purify blood stem cells by a simple one-step procedure. Establishment of this goal will have major contribution in the treatment of any disease which requires therapeutic measures such as stem cell transplantation and gene therapy. Proposed commercial applications: Stem cell purificatIon or enrichment has been actively explored, for the reasons that it has major clinical advantages in the treatment of diseases. Currently. a few good stem cell markers are available such that the combined usage of antibodies recognizIng these markers leads to encouraging but not optimal results. The limitations at the moment are due to complicated manipulations during the enrichment or purification processes which cause the enriched stem cells to undergo significant differentiation such that they lose their "stemness". In this regard, our findings that gp105 is highly specific for blood stem cells and that single gp105-sorted cells can repopulate the blood system have major implications. Based on these findings, our success In establishing a one- step procedure for enrichment or purification of blood stem cells will potentially provide a major breakthrough in treatment for diseases such as AIDS. cancer and various genetic and metabolic abnormalities In which stem cell transplantation and gene therapy are included as treatment modalities.