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Transactivation is an increased rate of gene expression triggered either by endogenous cellular or viral proteins - transactivators. These protein factors act in trans (i.e., intermolecularly). HIV and HTLV are just two of the many viruses that encode transactivators to enhance their own gene expression. These transactivators can also be associated with cancer if they start interacting and increasing expression of a cellular proto-oncogene. HTLV, for instance has been associated with causing leukemia primarily through this process. Its transactivator (named tax) can interact with p40, causing overexpression of IL-2, IL-R, GM-CSF and the transcription factor fos. HTLV infects T-cells and so, with increased expression of these stimulatory cytokines and transcription factors, leads to uncontrolled proliferation of T-cells and hence lymphoma.

Transactivation is also a technique used in molecular biology to control gene expression by stimulating transcription. During transactivation, the transactivator gene and special promoter regions of DNA are inserted into the genome at areas of interest. The transactivator gene expresses a transcription factor that binds to specific promoter region of DNA. By binding to the promoter region of a gene, the transcription factor causes that gene to be expressed. The expression of one transactivator gene can activate multiple genes, as long as they have the specific promoter region attached. Because the expression of the transactivator gene can be controlled, transactivation can be used to turn genes on and off. Also, if this specific promoter region is also attached to a reporter gene we can see when the transactivator is being expressed.

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