Inclusion Criteria:  Patients must be aged from 0-18 years old on entry to the study; Clinical indication of allogeneic or autologous hematopoietic stem cell transplantation; The conditioning protocol must include IV Bu formulations, Busulfex® (Otsuka Pharmaceutical), Busilvex® (Pierre Fabre Pharma) or other European Medicines Agency (EMA) or Food and Drugs Administration (FDA) approved generic formulations regardless of the administration schedule (q6h, q12h, or q24h) The expected length of time from recruitment to starting the conditioning regimen must be superior to 10 days; Informed written consent to participate in the study signed by the participant/parent  Exclusion Criteria:  • At least one of the drugs listed below scheduled to be administered in the Bu administration days up to 24h after the last dose of Bu, whenever a washout is not possible:  Metronidazol (required washout: 7 days) Nalidixic acid (required washout: 7 days) Phenytoin (required washout: 21 days) Itraconazole (required washout: 14 days) Ketoconazole (required washout: 7 days) Voriconazole (required washout: 7 days) Deferasirox (required washout: 7 days)