When the integrity of the 5' LTR is disrupted, the 3' LTR can act as a promoter, and this is the basis for one form of retroviral oncogenesis. Thus transcription from the 5' LTR interferes with the function of the 3' LTR as a promoter-enhancer. Experiments by Cullen et al. By progressively removing portions of a provirus and using an indicator gene downstream of the 3' LTR, the transcriptional activity from both LTRs could be assessed. Cellular restriction of LTR function can occur.
Transcriptional enhancers are tissue and cell specific in their function.
For retroviruses like MuLV, embryonal tissues and cells do not host viral gene expression whereas differentiated or adult tissues and cells express viral RNA to high levels. Changes in the structures of the binding sites in the LTRs can be correlated with tissue-specific expression patterns of mutant MLV and their induction of tumor formation in those tissues.