This study looks at two ways to use a medicine called abatacept to help prevent a condition called Graft Versus Host Disease (GVHD) after a bone marrow transplant. GVHD happens when transplanted cells attack the body. Patients will be split into two groups: one will get abatacept for a short time with a fake medicine (placebo), and the other will get it for a longer time. Everyone will get four doses at first, then some will get four more doses of abatacept while others get placebo. The study aims to see which way works better over two years. It will be done at many centers and is a double-blind trial, meaning neither patients nor doctors know who gets what medicine. This is a Phase 2 trial, meaning it's still testing how well and safe the treatment is.

• The study takes two years, with results checked after one year.
• Participants must be at least 2 years old, weighing over 10 kg, and not have certain health conditions.
• Patients will receive standard care plus abatacept or placebo.