which commonly regulate expression in mammalian genes. The addition of a methyl group to either histone tails or directly on DNA causes the nucleosome to pack tightly together restricting the transcription of any genes in this region. This process not only has the capabilities to inhibit gene expression, it can also increase the chance of mutations. Stephen Baylin observed that if promoter regions experience a phenomenon known as hypermethylation, it could result in later transcriptional errors, tumor suppressor gene silencing, protein misfolding, and eventually cancer growth. Baylin et al. found methylation inhibitors known as azacitidine and decitabine. These compounds can actually help prevent cancer growth by inducing re-expression of previously silenced genes, arresting the cell cycle of the tumor cell and forcing it into apoptosis. There are further clinical trials under current investigation regarding treatments for hypermethylation as well as alternate tumor suppression therapies that include prevention of tissue hyperplasia, tumor development, or metastatic spread of tumors. The team working with Wajed have investigated neoplastic tissue methylation in order to one day identify early treatment options for gene modification that can silence the tumor suppressor gene. In addition to DNA methylation, other epigenetic modifications like histone deacetylation or chromatin-binding proteins can prevent DNA polymerase from effectively transcribing desired sequences, such as ones containing tumor suppressor genes. Clinical significance Gene therapy is used to reinstate the function of a mutated or deleted gene type. When tumor suppressor genes are altered in a way that results in less or no expression, several severe problems can arise for the host. This is why tumor suppressor genes have commonly been studied and used for gene therapy. The two main approaches used currently to introduce genetic material into cells are viral and non-viral delivery methods. Viral methods The viral method of transferring genetic material harnesses the power of viruses. By using viruses that are durable to genetic material alterations, viral methods of gene therapy for tumor suppressor genes have shown to be successful. In this method, vectors from viruses are used. The two most commonly used vectors are adenoviral vectors and adeno-associated vectors. In vitro genetic manipulation of these types of vectors is easy and in vivo application is relatively safe compared to other vectors. Before the vectors are inserted into the tumors of the host, they are prepared by having the parts of their genome that control replication either mutated or deleted. This makes