Largely gutted-out versions of HIV or related viruses used to deliver genes into cells. These RNA-based viruses include a reverse transcriptase that, once inside cells, makes a DNA copy of its RNA and integrates it into the host DNA. Because they integrate in the genome, they offer long-term expression, which may be desired, but also risk causing problems if they insert into a bad location (such as the middle of a gene). Integrase-deficient versions are being developed and tested for use in applications not requiring long-term expression (e.g., vaccines). A couple key features are that they can infect non-dividing and slowly-dividing cells and can deliver up to ~ 9 kb (kilobase pairs) of DNA.
