This study investigates the effectiveness and safety of Maraviroc (an oral medication given twice daily given in addition to the standard GVHD prophylaxis) in preventing Graft versus Host Disease (GVHD) in patients undergoing non-myeloablative allogeneic stem-cell transplantation (SCT). Subjects will receive Maraviroc bid (in addition to standard GVHD prophylaxis) beginning after the last dose of the chemotherapy conditioning regimen until day 30 after stem-cell infusion.