Sensitization to minor histocompatibility antigens by prior transfusions of blood products can increase this risk. The rapid development of complete chimerism in NK-cells and T-cells seems to play an important role to achieve sustained engraftment specifically in patients transplanted with a dose reduced preparative regimen. Therefore, it is important to elucidate the development of post transplant chimerism in different cell subpopulations. This will allow following and documenting proper engraftment, and will detect early hints of ongoing graft rejection.