Retroposons are repetitive DNA fragments which are inserted into chromosomes after they had been reverse transcribed from any RNA molecule. In contrast to retrotransposons, they never encode Reverse Transcriptase (RT). Therefore, they are non-autonomous elements with regard to transposition activity. (compare Transposons)

Non-LTR retrotransposons (such as the human L1 elements) are sometimes falsely referred to as retroposons. Retroposition accounts for approximately 10,000 gene-duplication events in the human genome, of which approximately 10% are functional. Such genes are called retrogenes and represent a certain type of retroposons. A classical event is the retroposition of a spliced pre-mRNA molecule of the c-scr gene into the proviral ancestor of the Rous Sarcoma Virus (RSV). The retroposed c-src pre-mRNA still contained a single intron and within RSV is now referred to as v-src gene.

References

Emerson JJ, Kaessmann H, Betran E, Long M (2004) Extensive gene traffic on the mammalian X chromosome. Science 303:537-540