Retrovirus, any of a group of viruses that belong to the family Retroviridae and that characteristically carry their genetic blueprint in the form of ribonucleic acid (RNA). Retroviruses are named for an enzyme known as reverse transcriptase, which was discovered independently in 1971 by American virologists Howard Temin and David Baltimore. Reverse transcriptase transcribes RNA into deoxyribonucleic acid (DNA), a process that constitutes a reversal of the usual direction of cellular transcription (DNA into RNA). The action of reverse transcriptase makes it possible for genetic material from a retrovirus to become permanently incorporated into the DNA genome of an infected cell; the enzyme is widely used in the biological sciences to synthesize genes.

Retroviruses cause tumour growth and certain cancers in animals and are associated with slow infections of animals, such as equine infectious anemia. In humans, a retrovirus known as human T-cell lymphotropic virus type 1 (HTLV-1) causes a form of cancer called adult T-cell leukemia (ATL). It can also cause a neurodegenerative condition known as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). A closely related virus named HTLV-2 is associated with relatively mild neurological disorders but has not been identified as a causative agent of human disease. As many as 20 million people worldwide are thought to be infected with HTLVs, but only a small percentage of infected individuals actually develop ATL or HAM/TSP. The retrovirus known as human immunodeficiency virus (HIV) causes acquired immunodeficiency syndrome (AIDS) in humans. HIV is closely related to simian immunodeficiency virus (SIV), a retrovirus found in chimpanzees and gorillas. Retroviruses are a virus family of considerable medical and veterinary importance. Additionally, it is now clear that endogenous retroviruses (ERVs) comprise significant portions of vertebrate genomes. Until recently, very little was known about the deep evolutionary origins of retroviruses. However, advances in genomics and bioinformatics have opened the way for great strides in understanding. Recent research employing a wide variety of bioinformatic approaches has demonstrated that retroviruses evolved during the early Palaeozoic Era, between 460 and 550 million years ago, providing the oldest inferred date estimate for any virus group. This finding presents an important framework to investigate the evolutionary transitions that led to the emergence of the retroviruses, offering potential insights into the infectious origins of a major group of pathogenic viruses. Retrotransposons tune immune reactivity in differentiated cells because when they are transcribed, their nucleic acids can be viewed as non-self leading to innate immune sensing. Most retrotransposons, however, are subject to transcriptional regulation by a multitude of epigenetic pathways, which have coevolved with them for millions of years. While a lot is known about the epigenetic control of retrotransposons in germ cells and early embryos, surprisingly little is understood about these pathways in adult tissues, particularly in human cells. Recent evidence suggests that retrotransposon repression persists in differentiated cells and is dynamic. Future insight into this topic may teach us how to reactivate or silence specific retrotransposon families, to promote anti-tumor immunity or dampen autoimmunity through epigenetic modulation.