Pilot Trial of Clofarabine Added to Standard Busulfan and Fludarabine for Conditioning Prior to Allogeneic Hematopoietic Cell Transplantation

The purpose of this study is to find out what effects, good and/or bad, the addition of clofarabine, a new chemotherapy agent, to a standard busulfan and fludarabine conditioning treatment has. The study will also look at what causes some people to have high drug levels of these medications in their body compared to other people that may have low drug levels even if they all receive the same dose of medication.