The main purpose of this study is to assess the effects of cyclophosphamide (cytoxan) in the post transplant setting to prevent onset of acute graft-versus-host disease (GVHD). The primary objective is to determine the incidence of grade II-IV acute GVHD following Allogeneic (allo) Hematopoeitic Cell Transplant (HCT) using post-transplant cyclophosphamide (cytoxan) for patients with human leukocyte antigen (HLA) matched unrelated (MUD) and mismatched unrelated (MMUD) donors. Other objectives for this study will be the determination of disease-free survival (DFS) and overall survival (OS) following allo HCT and assess the safety of post-transplant cyclophosphamide (cytoxan) for MUD and MMUD transplantation. Disease recurrence and time to recurrence in patients receiving post-transplant cyclophosphamide compared to historical control without post-transplant cyclophosphamide (cytoxan) will also be evaluated. Other objectives will be to determine the time of onset, severity, responsiveness to treatment, organs involved of acute and chronic GVHD as well as observation of Immune Reconstitution over time.