Cancer is known to be a genetic disease, which is caused by abnormalities of oncogenes and/or tumor-suppressor genes. Genetic abnormalities include mutation, deletion or amplification of DNA sequences. Based on the findings of the genetic abnormalities in malignant tumors, many molecular-targeting agents, e.g. Bcr-Abl kinase inhibitors and EGFR kinase inhibitors, have been developed and approved. In addition to genetic abnormalities, epigenetic abnormalities, e.g. DNA methylation, histone methylation and histone acetylation, are also involved in carcinogenesis and tumor development. Based on the findings of the epigenetic abnormalities in malignant tumors, the novel epigenetic molecular-targeting agents, e.g. DNA(DNMT) methyltransferase inhibitors and histone deacetylase (HDAC) inhibitors, have also been developed and approved. Moreover, histone methyltransferase(HMT) inhibitors and histone demethylase(HDM) inhibitors have been also discovered and some agents are in clinical trials.