A study of patients undergoing haematopoietic stem cell transplantation, a procedure in which patients are infused with stem cells from a donor, resulting in a new immune system that eliminates cancer or replaces diseased bone marrow. This study aims to develop new blood tests that predict the onset of acute graft-versus-host disease (aGvHD) and leukaemia relapse, two life-threatening complications that frequently limit the success of treatment. Predictive tests would allow doctors to individualise prophylaxis and intervene early to abort complications before they develop. The study will also create a large collection of clinically annotated blood samples from 300 transplant recipients to support future research and provide a resource to the transplant research community.