If bone marrow or stem cells from a donor are used, the transplant is called an allogeneic transplant – or syngeneic transplant if the donor is an identical twin. In an allogeneic transplant, the new marrow or stem cells infused into the patient must match the genetic makeup of the patient's own marrow or stem cells as perfectly as possible. Special blood tests are conducted to determine whether or not the donor's tissue will match the patient's. If the donor's tissue is not a good genetic match, it will perceive the patient's body as foreign material to be attacked and destroyed. This condition is known as graft-versus-host disease (GVHD) and can be life-threatening. Alternatively, the patient's immune system may destroy the new bone marrow or stem cells. This is called graft rejection. Special programs designed to reduce recovery time, prevent infections and deal with GVHD are available at Siteman.