Subjects who have had an allogeneic Hematopoietic Stem Cell Transplant (HSCT) will enter treatment with AC220 between 30 to 60 days after receiving allogeneic HSCT. AC220 will be administered every day, with 28 consecutive days defined as a treatment cycle. Subjects may receive up to 24 continuous treatment cycles. Subjects will have study visits each week for the first 2 cycles, and then on Day 1 of each cycle after that.