 One of the major side effects of any stem cell transplant is a condition known as graft vs. host disease or GVHD. GVHD is an immune reaction caused by certain cells from the transplanted stem cells called T-lymphocytes or T-cells. The T-cells from your donor may see your organs as foreign and attack them. New ways to remove the T-cells from the stem cells before the transplant are being used to try and prevent GVHD. In some studies, the removal of T-cells from the stem cells has been successful for many patients in preventing both short-term acute and long-term chronic forms of GVHD. However, the removal of T-cells may increase the chance that the new bone marrow developing from the stem cells will be rejected or will not function well. Rejection of the transplant means that some of your own cells have survived the chemo and radiation therapy, and are attacking the new bone marrow cells. This condition can be lifethreatening because of an increased risk of infections and bleeding and would require your getting more treatment and additional stem cells. Studies like this one are designed to find better ways to avoid GVHD without increasing the risk of other problems such as graft rejection. 