Who is the best alternative allotransplant donor?

Assuming that most physicians will chose an HLA-identical sibling as the best allotransplant donor, the question arises who is the best alternative donor when an HLA-identical sibling is unavailable? The most commonly used alternative donors are HLA-identical or -mismatched unrelated donors, HLA-matched or -mismatched umbilical cord blood donor or a related, HLA-haplotype-matched related donors. Each alternative donor option has advantages and disadvantages. We discuss selected aspects of these issues based on data from randomized clinical trials and observational databases. However, because there are limited data to address specific clinical settings, quantification of expert opinion is sometimes needed.