 Stage 1 Cross-Sectional Study: For the sample size calculation the Primary end-point is to test the ability of ProALL miRs measured in blood to differentiate B-ALL patients at remission from patients at relapse. The sample size calculation is according to Sampling: comparison of proportions of MedCalc MedCalc Statistical Software version 16.1 MedCalc Software bvba, Ostend, Belgium; 2016. Type 1 error was 0.05 and Type 2 error was 0.20. Data for the sample size calculation was obtained from the DCOG cohort in Avigad et al., 2016, which was chosen as a model for the feasibility study due to its small size n32. Cross sectional analysis of the DCOG cohort showed that there were significantly more patients with relapse event p0.001, Fisher Exact test positive for at least one miR 78 compared to patients with no relapse event 13. 