Epigenetics is concerned with heritable changes in gene expression without alteration of the coding sequence. Epigenetic modification of chromatin includes methylation of genomic DNA as well as modification of chromatin-associated proteins, in particular, histones. Epigenetic deregulation affects several aspects of tumor cell biology, including cell growth, cell cycle control, differentiation, DNA repair, and cell death. This raises the strong possibility that reversing deregulated epigenetic mechanisms may be an effective treatment for leukemia and cancer.