RecruitingDiagnostic testA Study to Evaluate Next-Generation Sequencing (NGS) Testing and Monitoring of B-cell Recovery to Guide Management Following Chimeric Antigen Receptor T-cell (CART) Induced Remission in Children and Young Adults With B Lineage Acute Lymphoblastic Leu...Background: Chimeric antigen receptor T-cell (CART) therapy is a form of immunotherapy which can be used to treat people with relapsed B-ALL. For those who achieve remission after CART alone, it may cure up to 50% of people who receive this therapy. However, for people who relapse after CART, it can be hard to achieve remission again. In patients where CART fails, stem cell transplant (HCT) can be used to prevent relapse and achieve cure. But HCT can cause serious side effects. Better testing is needed to distinguish people who can be cured with CART alone from people who may also need to have HCT. Objective: To see if the use of a series of blood and bone marrow tests at regular intervals can help monitor for B-ALL relapse after CART therapy. Eligibility: People aged 1 to 25 years with B-ALL who have had CART therapy within the past 42 days. They must never have had a blood stem cell transplant; they must also have no measurable blood cancer cells. Design: Participants will visit the clinic every 2 weeks starting 42 days after they receive CART therapy. Each visit will be about the same amount of time as a regular clinic visit. about 8 hours. Participants will have blood drawn for testing on each visit. Bone marrow biopsy/aspirate will be done during 4 of the visits at routine timepoints after CART. A needle will be inserted to draw a sample of tissue from inside the bone in the hip. A small amount of blood and tissue will be tested with ClonoSEQ and to evaluate for normal B-cells side by side with the standard tests. The combined testing may help determine whether participants are eligible for HCT and/or at risk of relapse after CART. Participants will be in the study for 2 years.