Conditions associated with increased occurrence of graft failure include defects in the bone marrow microenvironment, immunological disturbances or imbalances between donor and recipient (HLA disparity, alloimmunization with anti-HLA antibodies, ABO mismatching in the donor/recipient pairs, etc.), low infused hematopoietic stem cell dose, T-cell depleted (TCD) grafts, reduced-intensity conditioning regimens (RIC), drug toxicity (myelosuppressives such as ganciclovir) and infections, especially of viral origin, such as those caused by cytomegalovirus (CMV) [16, 35–37]. The gene discussed is ABO; the disease is infection.