Methylation is a change that occurs to DNA that has an effect on how genes are used in human cells. It is very common in leukemias for methylation to happen abnormally. Decitabine is a new drug that blocks DNA methylation. At low doses (such as those used in this study), decitabine blocks proteins important in abnormal DNA methylation, which may, in turn, allow leukemia cells to die and disappear.