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

Inclusion Criteria:

• Patients who had mycophenolate treatment at least 3 days for immunosuppressant after allogenic hematopoietic stem cell transplantation
• Patients age <18 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 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
• Patients with hypoxanthine-guanine phosphoribosyltransferase deficiency(HGPRT) such as Lesch-Nyhan or Kelley-Seegmiller syndrome)