ALLOGENEIC TRANSPLANT PROCEDURE: Patients will receive a reduced-intensity conditioning regimen of cyclophosphamide, fludarabine and low-dose Total Body Irradiation (TBI), a regimen that has been used successfully in patients receiving haploidentical allografts. Marrow will be used as the graft source to reduce the risk of GVHD. GVHD prophylaxis will consist of post-transplant high-dose cyclophosphamide followed by the combination of tacrolimus and enteric coated mycophenolic acid. Supportive care includes the use of N-acetyl cysteine infusions to reduce the risk of sinusoidal liver injury from cyclophosphamide; prophylaxis with ursodiol to prevent cholestatic liver disease; and antimicrobial drugs as prophylaxis and preemptive treatment for infections by bacteria, fungi, herpes viruses, and Pneumocystis jiroveci. Tissue and blood samples will be archived for future studies and evaluation of immune reconstitution at predefined intervals.