Prior to NST, all patients will receive trimethoprim/sulfamethoxazole (10 mg/kg/d trimethoprim) in days -10 to -2, acyclovir (500 mg/m2 x 3/day) from days -6 until day +100, and allopurinol (300 mg/day) on days -10 to -1. Administration of trimethoprim/sulfamethoxazole (twice weekly) will be reinstituted after recovery from neutropenia as a preventive measure against pneumocystis carinii infection. Starting on day -8, patients are monitored with a pp65 antigenemia/CMV PCR on a weekly basis to detect cytomegalovirus (CMV). Two consecutive positive PCR results or one antigenemia with more then one cell positive for pp65 serve as an indication for replacing acyclovir with ganciclovir 10 mg/kg/day until minimum of two negative tests are obtained.