Epigenetics refers to the study of heritable changes in gene expression that occur without a change in DNA sequence. DNA methylation and histone modification (methylation, acetylation, deacetation, and phosphorylation) are epigenetic mechanisms which regulate gene expression. DNA methylation down-regulates gene activity while modification of the histone can be carried out by multiple mechanisms leading to either the upregulation or down-regulation of the associated gene. Epigenetic abnormalities have been found to be causative factors in cancer. Different methods can be used to assess epigenetic changes. HDAC inhibitors are the main classes of epigenetic drugs that target these epigenetic abnormalities.