Determining the Mechanisms of Loss of CAR T Cell Persistence

This research study is investigating why CAR-T cell therapy — a treatment that reprograms your immune cells to fight cancer — sometimes stops working over time in children and young adults with acute lymphoblastic leukemia (ALL). **You may be eligible if...** - You are 25 years old or younger - You have relapsed or treatment-resistant ALL (a blood cancer) - You are scheduled to receive the CAR-T therapy called tisagenlecleucel (Kymriah) as part of your standard care - You or your guardian can provide written informed consent **You may NOT be eligible if...** - You are receiving a different CAR-T therapy on a separate clinical trial - There is a medical reason you cannot follow the study procedures Talk to your doctor to see if this trial is right for you.