Clofarabine, similar to Fludarabine, is known to have a stronger anti-tumor effect than Fludarabine and has shown promise in treating aggressive acute leukemias. In addition, evidence is that it is well-tolerated with manageable side effects especially in older subjects. Thus replacing Fludarabine with Clofarabine in a full-intensity transplant regimen, Clo/BU4 may provide a regimen with increased anti-tumor activity without adding significant risks of toxicity.