Mismatched unrelated donor transplantation.

There are now more than 25 million volunteer donors registered worldwide for patients in need of a life-saving hematopoietic cell transplant to cure blood disorders. Although a human leukocyte antigen (HLA)-matched donor remains the preferred stem cell source for transplantation, the use of a donor with limited HLA mismatching may be considered. Significant advances in clinical and basic research have been instrumental in furthering the understanding of donor-recipient HLA mismatches that are better tolerated than other mismatches. An increased appreciation of the role of regulatory region variation that affects the level of HLA expression provides new approaches for the selection of HLA-mismatched donors.