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