This is a recent article from the Journal of American Medical Association:

The Next Frontier for Stem Cell
Finding a Donor for All
Lucy A. Godley, MD, PhD
Koen van Besien, MD, PhD
JAMA. 2010;303(14):1421-1422 (doi:10.1001/jama.2010.413)

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 highresolution
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.
Given the inability to identify HLA-matched donors for
all patients, this Commentary discusses several strategies to
increase the pool of acceptable mismatched donors for HSCT.
Sources of HSCs for these transplants include (1) blood or
bone marrow HSCs from adult unrelated donors; (2) umbilical
cord blood (UCB) stem cells; (3) blood or bone marrow
HSCs from adult mismatched relatives (haploidentical
transplantation); and (4) combinations of various donor
sources, in particular haplo-cord transplantation.