Stem cell transplantation is a life-saving therapy for various severe diseases. During a transplantation, the patient’s blood producing stem cells are replaced by those of a healthy donor. Depending on the source of the donor cells (cord blood, bone marrow of a sibling or adult donor), the age and number of transplanted cells may differ. Particularly in young recipients, it is important to determine how these differences affect short- and long- term blood production. My laboratory uses a method to study individual stem cells in great detail. We will use this method on samples of children who received a stem cell transplant for high-risk leukemia. We will study the function of the transplanted stem cells before and after transplantation, and determine how they communicate with the bone marrow environment of the patient. Our study will contribute to new methods to enhance blood regeneration, to improve the survival of stem cell transplantation recipients and to reduce its side effects.