Allogeneic (genetically different, same species) hematopoietic stem cell transplantation (HSCT) is a well-established therapy for a variety of hematological malignancies, inherited disorders of metabolism, histiocytic and granulomatous disorders, adjuvant therapy in kidney transplant, and recently, multiple sclerosis. Syngeneic transplantation (between identical twins) or autologous (from the patient’s own cells) represent the optimal form of HSCT and, unlike other allogeneic donors, does not carry a risk of GVHD. Even with sibling donors, who are more likely than unrelated donors to be HLA-matched, 25–35% of recipients develop GVHD. Transplant recipients with GVHD have a reduced quality of life and increased risks for long-term morbidity and mortality in comparison to recipients who do not develop this disease.