Steroids are the standard initial treatment of acute GVHD, with approximately 50% of the patients either free of disease or requiring no further therapy. In the remaining patients, the GVHD either does not respond or it comes back during the tapering of steroids. These patients have a much worse prognosis with a mortality rate greater than 70%. Studies using additional agents such as antithymocyte globulin (ATG), monoclonal antibodies, and anti-lymphocyte globulin showed no improvement over the use of steroids alone. This leads the investigators to look for new immunosuppressive agents that can reduce the risk and severity of acute GVHD.