Cancer development is a multi-step process with mutations in proto-oncogenes and tumor suppressor genes playing a well-defined role.  In this regard, cancer is a genetic disease. However, cellular mechanisms that control the differentiation state, survival and self-renewal of cancer cells are often independent of mutations in DNA sequence and are increasingly recognized as critical for tumor development and progression. These epigenetic mechanisms are largely a result of the multitude of chromatin modifications that control gene expression. Since epigenetic changes are largely reversible, there is significant hope that therapies targeting these modifications may be particularly effective anti-tumor agents.