The ability to expand mobilized CD34+ cells in vitro using various cytokine combinations and culture conditions will be examined using flow cytometry, in vitro colony assays and by quantitating the concentration of cells that are able to establish hematopoiesis in immunodeficient mice. Both unexpanded and expanded CD34+ cells will be used in experiments designed to investigate their suitability as targets for retroviral-mediated gene transfer. Gene transfer efficiency and functional expression of transferred sequences will be assessed in the differentiated progeny of CD34+ cells.