Sirolimus With Tacrolimus for Graft-vs-Host Disease Prophylaxis After Un-Related Stem Cell Transplantation

* Therapy to prevent GVHD will consist of an infusion of tacrolimus intravenously and sirolimus orally once daily starting 3 days before stem cell infusion. This will take place in the hospital where the patient will remain for the duration of the transplant. * Sirolimus will continue for approximately 100 days at a stable dose, then it will be tapered slowly over the course of weeks to months to prevent a flare in GVHD. * Patients will be seen in the clinic weekly for the first 2 months after discharge from the hospital. If GVHD is present, tapering schedule will be slower and based on the patient's clinical condition. * Tacrolimus will also be given orally after the patient is discharged and will be tapered on the same schedule as sirolimus. * During the year following stem cell transplant, blood tests will be performed to evaluate the immune system and graft versus host disease.