A retrovirus is a virus which has a genome consisting of RNA. It relies on reverse transcriptase to perform a kind of reverse transcription[?] of its genome from RNA into DNA for insertion by integrase into the host's genome. The virus itself is just a storage form for its RNA; the reverse transcription takes place in the host's cytosol. A retrovirus' genome integrated into the host's genome is called a provirus.

The retrovirus genome contains at least three genes:

• gag codes for core and structural proteins of the virus.
• pol codes for reverse transcriptase.
• env codes for the virus hull proteins.

There are three known retrovirus categories :

• Oncovirinae cause sarcomas[?] and leukaemias (e.g., Rous Sarcoma Virus[?]). They contain an onc gene which makes them oncogenic[?].
• Lentivirinae cause slow progressive degenerative disorders (e.g., HIV).
• Spumavirinae with unknown effects.