Nowadays, it became evident that the onset of cancer and its progression may not occur only due to genetic mutations but also because of changes in the patterns of epigenetic modifications. In contrast to genetic mutations, which are almost impossible to reverse, epigenetic changes are potentially reversible. This implies that they are susceptible to pharmacological interventions. Therefore, a lot of work in recent years has focussed on the development of small molecule enzyme inhibitors like DNA-methyltransferase inhibitors or inhibitors of histone-modifying enzymes. Today, several epigenetic drugs are already approved by the FDA for cancer treatment and around ten histone deacetylase (HDAC) inhibitors are in clinical development. The purposes of this thesis are • Understand about mechanisms of action of HDAC inhibitors • Drugs are already approved by the FDA for cancer treatment and histone deacetylase (HDAC) inhibitors are in clinical development