Two families of animal viruses utilize reverse transcriptase (RT) in the replication of their genome, the Retroviridae and the Hepadnaviridae.

The retroviruses have been intensively studied for years because researchers discovered early that avian Retroviruses have the ability to induce leukaemia and sarcomas in chickens. The study of these viruses led to the discovery of cellular oncogenes, of RT, and of mechanisms that regulate cycling of the animal cell, and several Nobel prizes have been awarded for work with the avian retroviruses.

Although clearly important for our understanding of biology, for many years after their discovery Retroviruses were in some ways biological curiosities because no human disease was known to be associated with Retroviral infection. This changed with the discovery of human T-cell leukemia viruses, now known as primate T-lymphotropic viruses (PTLV), which cause leukemia in man. More recently, the appearance of Human Immunodeficiency Virus (HIV) and of acquired Immunodeficiency Syndrome (AIDS) in the human population has dramatically altered our understanding of the disease-causing potential of retroviruses.