This is an investigator-initiated clinical trial to analysis population pharmacokinetic characteristics and investigate appropriate pediatric dose of Cyclophosphamide in pediatric hematopoietic stem cell transplantation patients.

Inclusion Criteria:

• Patients who had cyclophosphamide treatment for immunosuppressant after allogeneic hematopoietic stem cell transplantation
• Patients age <19 years
• Written Study Informed consent and/or assent from the patient, parent, or guardian

Exclusion Criteria:

• Known hypersensitivity to mycophenolate mofetil or similar class of drug substance
• Patients in a medically critical condition such as severe infection or unstable vital signs
• Any condition that would, in the Investigator's judgment, interfere with full participation in the study
• Subjects who are pregnant or breast-feeding
• Subjects with psychiatric conditions that may interfere with the study
• Subjects who have a possibility of the disease getting worse as a treatment for clinical trials