 2.2. Wider application of allogeneic HCT in patients with ALL, however, is impeded by limited donor availability. Less than a third of patients who need allogeneic HCT will have a HLA-matched sibling who can donate hematopoietic cells. For those patients who do not have an HLA-matched donor in the family, provided that they do not carry rare or private HLA-haplotype, HLA-matched unrelated donor can be found.3 The chance of finding a willing unrelated donor in Korea is about 50. On the other hand, nearly all patients who are in the need of allogeneic HCT have at least one HLA-haploidentical familial member, who is most willing to give hematopoietic cells immediately, not only for the initial transplantation, but also for the subsequent additional donations, if those became necessary. Early attempts to transplant allogeneic hematopoietic cells across the barriers of HLA-haplotype difference was met with high frequencies of engraftment failure and severe graft-versus-host disease GVHD.4, 5 Depletion of donor T cells from the graft before HCT decreased the frequency and severity of GVHD. However, it resulted in increased graft failure, delayed immune reconstitution, and increased fatal infections.6-8 Further efforts to improve the outcomes of HLA-mismatched familial donor HCT included use of polyclonal9, 10 or monoclonal antibodies11, 12 against T cell as a part of conditioning regimen in vivo-T cell depletion and incorporation of the concept of feto-maternal immune tolerance in the donor selection process among several available HLA-haploidentical family members.13 