Ruxolitinib With Tacrolimus and Methotrexate for the Prevention of Graft Versus Host Disease in Pediatric and Young Adult Patients Undergoing Allogeneic Hematopoietic Cell Transplant for Acute Myeloid Leukemia, Acute Lymphoblastic Leukemia, or Myelodysplastic Syndrome

This trial is testing whether adding Ruxolitinib (a drug that reduces inflammation) to the standard medications Tacrolimus and Methotrexate can better prevent graft-versus-host disease (GVHD) — a serious complication where donated immune cells attack the patient's body — in children and young adults receiving bone marrow transplants for leukemia or a blood disorder called myelodysplastic syndrome. **You may be eligible if:** - You are 2–22 years old and weigh at least 25 kg - You have been diagnosed with AML (acute myeloid leukemia), ALL (acute lymphoblastic leukemia), or MDS (myelodysplastic syndrome) - You are in complete remission from your leukemia - You have a suitable matched bone marrow donor (related or unrelated) - Your physical functioning is adequate (Karnofsky/Lansky score at least 60%) **You may NOT be eligible if:** - You have had a prior bone marrow transplant - You have had an autologous (self) stem cell transplant within the past year - You have active tuberculosis or a history of blood clots (heart attack, stroke, DVT, pulmonary embolism) within 6 months - You have active HIV, hepatitis B, or hepatitis C - You have another active cancer besides the one being treated - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.