Allogeneic hematopoietic stem cell transplantation is a life-saving procedure for hematopoietic malignancies, marrow failure syndromes, and hereditary immunodeficiency disorders. However, wide application of this procedure is limited by availability of suitably HLA-matched adult donors. Umbilical cord blood (UCB) has being increasingly used as an alternative hematopoietic stem cell source for these patients. To date, over 6000 UCB transplant procedures in children and adults have been performed worldwide using UCB donors. Broader use of UCB for adult patients is however limited by the available infused cell dose. This has prompted intensive research on ex vivo expansion of UCB stem cells and