 Stage 1-Cross-Sectional Study: Blood samples will be collected from relapsed pediatric B-ALL patients and B-ALL patients in remission. Three Blood samples will be collected from each patient in tubes in order to interpret the best blood source for measuring miR-451, miR-151-5p and miR-1290. The sources that will be tested are serum, peripheral blood lymphocytes and whole blood. The level of the miRs in blood will be compared between relapsed B-ALL patients and B-ALL patients in remission n30. If the Stage 1 Cross-Sectional study is successful, the investigators will continue the clinical trial to the Stage 2 Prospective Monitoring study. 