Stem cell transplantation is an important pillar in the treatment of leukemia. An important prerequisite for a successful treatment is the matching of tissue characteristics between donor and recipient. Minor histocompatibility antigens (MiHAs), arising from small genetic differences between donor and recipient and recognized by the immune system, are the key factors that determine how successful a stem cell transplantation is and how serious the side effects are. In this project, mass spectrometry will be used to identify new, previously unknown MiHAs and to characterize the immune response directed against them with the aim to minimize adverse reactions in stem cell transplantation and to increase treatment success.