Why do patients relapse after transplant?

Why do patients relapse after transplant? So the idea of a bone marrow transplant really is we're taking your immune system, which doesn't recognize your leukemia, as being something that shouldn't be there, and we're replacing it with a new immune system that hopefully is going to recognize your leukemia as something that shouldn't be there and fight your leukemia, essentially. And so why do patients relapse when we've given this new immune system that's intended to fight the leukemia? And what we're learning more and more is that it has to do with how the leukemia subsequently evolves to evade that new immune system. And it certainly has led us to wonder, are there ways that we can then subsequently manipulate that new immune system to either prevent that from happening or once it has happened, is there a way that we can manipulate the new immune system, the transplanted immune system, to attack the leukemia again and put the patient back into remission? I think that it turns out more and more, we think, is probably the reason why patients relapse after a bone marrow transplant is because the leukemia changes in some way to evade that new immune system that's there to prevent the leukemia from coming back.