An RNA-containing virus that converts its RNA into DNA by means of the enzyme reverse transcriptase; this enables it to become integrated into its host’s DNA as a provirus. Some retroviruses can cause cancer in animals: they contain oncogenes (cancer-causing genes), which are activated when the virus enters its host cell and starts to replicate. Also, their insertion into the host genome may activate growth-promoting host genes abnormally. The special properties of retroviruses make them useful as vectors for inserting genetic material into eukaryotic cells. The best-known retrovirus is HIV, responsible for AIDS in humans.

https://web.stanford.edu/group/nolan/_OldWebsite/tutorials/tutorials.html Series of tutorials about retroviruses and their applications, compiled by the Nolan Lab of Stanford University