HC is a recognized complication of allogeneic transplant therapy. Late onset HC, occurring after engraftment, is due almost exclusively to reactivation of the polyoma BK virus (BKV). Other important risk factors associated with HC include Busulfan-based conditioning, acute GVHD, HLA mismatched transplants, and use of bone marrow as the stem cell source. TBI-based conditioning, prior to myeloablative allogeneic transplant, has been associated with significantly less HC than Busulfan-based conditioning in both retrospective and prospective randomized trials.