 While stem cell transplantation has proven an effective means of treating a wide variety of diseases involving hematopoietic stem cells and their progeny, a shortage of donors has proved a major impediment to the widest application of the approach. Until recently, only MHC identical donors could be used with safety. Such donors were originally siblings or other closely related family members. Over the past decade, the growth of allogeneic donor panels has allowed transplantation with stem cells obtained from a volunteer donor panel. 