Editing Retrovirus (section)

==Provirus==
The DNA formed after reverse transcription (the provirus) is longer than the RNA genome because each of the terminals have the U3 - R - U5 sequences called long terminal repeat (LTR). Thus, 5' terminal has the extra U3 sequence, while the other terminal has the U5 sequence.<ref name="John Wiley & Sons"/> LTRs are able to send signals for vital tasks to be carried out such as initiation of RNA production or management of the rate of transcription. This way, LTRs can control replication, hence, the entire progress of the viral cycle.<ref>{{cite book |last1=MacLachlan |first1=N. James |last2=Dubovi |first2=Edward J. |title=Fenner's Veterinary Virology |date=2011 |publisher=Academic Press |isbn=978-0-12-375159-1 |page=250 |edition=Fourth |url=https://www.sciencedirect.com/science/article/pii/B9780123751584000146 |access-date=6 May 2020}}</ref> Although located in the nucleus, the non-integrated retroviral cDNA is a very weak substrate for transcription. For this reason, an integrated provirus is a necessary for permanent and an effective expression of retroviral genes.<ref name=":0" />

This DNA can be incorporated into host genome as a provirus that can be passed on to progeny cells. The retrovirus DNA is inserted at random into the host genome. Because of this, it can be inserted into [[oncogene]]s. In this way some retroviruses can convert normal cells into cancer cells. Some provirus remains latent in the cell for a long period of time before it is activated by the change in cell environment.{{citation needed|date=November 2022}}