Research Question 3.1.1 Chimerism and post transplant outcome During the past 3 decades, SCT has become a well established treatment procedure for many malignant and hematopoietic disorders in children and adults.1-8 After transplantation, it has been of central interest whether the newly developed hematopoietic system is of recipient or donor origin. The investigations of the genotypic origin of post transplant hematopoiesis are called chimerism analysis. Originally, it was believed that complete donor hematopoiesis is essential to maintain engraftment after allogeneic SCT in humans.9 In the last decades, however, it became evident that donor and recipient hematopoiesis could coexist after allo-SCT in the recipient. This state of coexistence of hematopoietic cells is called mixed chimerism which might end in an "autologous recovery". In patients with refractory cytopenia SCT after myeloablative conditioning regimen allowed prompt and sustained engraftment in virtually all patients. In this disease relapse has become a very rare event. Consequently, transplant related mortality and long term squeals have become major obstacles yet to be overcome to improve the children's well being and the prognosis of the disease. In SCT with RIC, the reduction of early and late toxicity may be counterbalanced by delayed engraftment, graft rejection, mixed chimerism and GVHD.