The prospective clinical phase II protocol MC-FludT.16/NM is to be conducted to verify safety and efficacy of Treosulfan-based conditioning compared to Busulfan-based conditioning in paediatric patients. Based on the given clinical experience with either Treosulfan-based or Busulfan-based conditioning in combination with Fludarabine no increased risk for graft failure is expected in paediatric patients. A potential benefit for study patients is expected with respect to a probably low non-haematological toxicity of treatment compared to myeloablative TBI-based conditioning or high-dose Busulfan-based conditioning in combination with Cyclophosphamide.