Long terminal repeat

Long terminal repeats (LTRs) are sequences of DNA that repeat hundreds or thousands of times. They are found in retroviral DNA and in retrotransposons, flanking functional genes. They are used by viruses to insert their genetic sequences into the host genomes.

Example
For example, a retrovirus genome might contain the following features:

LTR -- PBS -- PSI -- gag -- pol -- env -- LTR LTR = U3RU5
 * LTR - Long terminal repeats
 * PBS - Primer binding site
 * PSI - packaging site
 * gag, pol, env - genes that code for proteins used in viral infection and reproduction

Transcription
The LTRs are partially transcribed into an RNA intermediate, followed by reverse transcription into complementary DNA (cDNA) and ultimately dsDNA (double-stranded DNA) with full LTRs. The LTRs then mediate integration of the retroviral DNA via an LTR specific integrase into another region of the host chromosome. The first LTR sequences were derived by A.P. Czernilofsky and J. Shine (Proc. Natl. Acad. Sci, USA, Vol 74, pp 1473-1477, 1977; and Nucleic Acids Res., Vol 8, pp 2967-2984, 1980).

Retroviruses such as Human Immunodeficiency Virus (HIV) use this basic mechanism.