Fludarabine is a chemotherapy drug used extensively in bone marrow transplantation. The goal
of this study is to determine what causes some children to have different drug concentrations
of fludarabine in their bodies and if drug levels are related to whether or not a child
experiences severe side-effects during their bone marrow transplant. The hypothesis is that
clinical and genetic factors cause changes in fludarabine drug levels in pediatric bone
marrow transplant patients and that high levels may cause severe side-effects.