Complex sentence: The post-transplant prognosis often includes acute and chronic graft-versus-host disease which may be life-threatening; however in certain leukemias this can coincide with protection against cancer relapse owing to the "graft versus tumor" effect.
Split into simpler sentences: The post-transplant prognosis often includes acute and chronic graft-versus-host disease that may be life-threatening. However in certain leukemias this can coincide with protection against cancer relapse owing to the "graft versus tumor" effect.