Subjects who are recruited in this study are LLA patient, who are treated for routine control to Cipto Mangunkusumo Hospital, who meet the inclusion criteria and do not meet the exclusion criteria.

On the day of the control patient, (usually the patient is asked for routine control at week 3 on mercaptopurin) routine blood tests are performed. Patients and parents were given an explanation of this study. If patients and parents are willing to take part in the study, they are asked to sign informed consent. Patients who meet the inclusion criteria and do not meet the exclusion criteria, are recorded for demographic data and 5 ml of blood taking. The blood is directly divided into three, 2 mL for hematologic examination and albumin level, 1 mL for genotyping, and 2 mL for examination of drug levels. Routine hematologic examination and albumin levels were performed in the Cipto Mangunkusumo hospital laboratory. For genotypic examination, blood samples were stored at -80C, until they were used for analysis. For examination of the drug levels the blood sample is centrifuged and the erythrocyte preparation is then stored at -80C, until it is used for analysis.

Inclusion Criteria:

• Acute lymphoblastic leukemia (ALL) patients, 1 until 18 years old, male or female, who come to Cipto Mangunkusumo Hospital, Jakarta, Indonesia
• Received 6MP chemotherapy at least 1 month maintenance phase
• Received maintenance phase treatment regimen
• Willing to participate in research, signed informed consent and obtained parental consent to participate in research

Exclusion Criteria:

• Patients are experiencing severe infections
• Patients receiving colony stimulating factor (CSF), allopurinol, mesalazine, olsalazine, and sulfasalazine

Pediatric patients with LLA who received 6-MP chemotherapy in the maintenance phase, who come to RSCM, and who meets the inclusion criteria and does not meet the exclusion criteria.