Current treatment strategies for patients with acquired or congenital disorders of hematopoiesis, immune system diseases, or high-risk or recurrent hematological malignancies often involve allogeneic hematopoietic stem cell transplantation (HSCT). Identifying HLA-identical siblings is often difficult because each full sibling has only a 25% chance of matching the patient. Constraints surrounding the siblings, such as the number of full siblings, their availability, and, increasingly, their health, limit the use of HSCT for many patients who might otherwise benefit. For such patients, searches of the National Marrow Donor Registry and other registries worldwide attempt to identify unrelated donors who match the patient molecularly for at least 4, and possibly 5, HLA loci (HLA-A, -B, -C, -DR, and possibly -DQ).1 Identifying HLA-identical donors using these high-resolution typing methods has resulted in marked improvements in outcomes of matched unrelated HSCT. But for many patients and their physicians, donor identification remains a major hurdle to the use of HSCT, especially for members of ethnic minority groups who often have rare HLA types not represented in the registries.