Retroviruses are viruses in which the genetic material consists of ribonucleic acid (RNA) instead of the usual deoxyribonucleic acid (DNA). Retroviruses produce an enzyme known as reverse transcriptase that can transform RNA into DNA, which can then be permanently integrated into the DNA of the infected host cells.

Many gene therapy treatments and experiments use disabled mouse retroviruses as a carrier (vector) to inject new genes into the host DNA. Retroviruses are rendered safe by adding, mutating, or deleting viral genes so that the virus cannot reproduce after acting as a vector for the intended delivery of new genes. Although viruses are not normally affected by antibiotics, genes can be added to retroviruses that make them susceptible to specific antibiotics.

As of 2002, researchers have discovered only a handful of retroviruses that infect humans. Human immunodeficiency virus (HIV), the virus that causes acquired immune deficiency syndrome (AIDS), is a...