Cord blood-derived CD34+ cells have proved superior to adult marrow or elicited peripheral blood in ex vivo cell and progenitor expansion. In addition, cord blood-derived cells with a phenotype and function of early stem cells (eg, long-term culture-initiating cells) can be expanded in vitro in the presence of a combination of cytokines by 15-to 20-fold over 7-14 days. These observations suggest that ex vivo expansion of cord blood CD34+ cells will permit improved engraftment of adults and will prove effective in retroviral-mediated gene transfection of early stem cell populations.