 - To determine the feasibility of allogeneic hematopoietic stem cell transplantation after a reduced-intensity conditioning regimen comprising fludarabine phosphate, busulfan, and anti-thymocyte globulin, in terms of donor engraftment, transplant-related mortality, and development of acute and chronic graft-vs-host disease, in pediatric patients with high-risk relapsed or refractory neuroblastoma. 