This is an investigator-initiated clinical trial to analysis melphalan pharmacokinetics in pediatric hematopoietic stem cell transplantation patients.

Inclusion Criteria:

• The patient who will receive a conditioning regimen including melphalan as a part of hematopoietic stem cell transplantation (HSCT).
• Age is below 19 at the time of diagnosis of a disease which is HSCT indication
• Written Study Informed consent and/or assent from the patient, parent, or guardian

Exclusion Criteria:

• Female patients who are pregnant or breast feeding
• Study treatment would deteriorate patient's disease.
• The patient may have problems with study participation due to a psychotic disorder.
• Any condition that would, in the Investigator's judgement, interfere with participation in the study