Retroviruses are RNA viruses that have been shown to cause cancers in diverse species, including chickens, mice, cats, and monkeys. The RNA genomes of these viruses are first reverse-transcribed into single and then double-stranded DNA (dsDNA) copies. This dsDNA called proviral DNA then integrates into the host genome. Subsequently, the host cell transcribes the proviral DNA in concert with the chromosomal DNA. This leads to the production of viral RNA and proteins that assemble at the host cell surface, forming new viral particles. When the viral genes or products interfere with host genes that regulate cell growth and progression, it leads to cancer progression.

Acute vs. non-acute retroviruses

Some retroviruses called acute viruses can cause cancer after short latent periods. For example, the Rous sarcoma virus can induce sarcomas in the host within three weeks of infection. In contrast, some non-acute retroviruses only cause cancer after long latent periods. Such non-acute viruses induce cancer in only some of the infected hosts. For example, Rous-associated virus-60 induces lymphomas in only 50% of the infected hosts after five to nine months of infection.

However, additional events and host factors such as immunosuppression, somatic mutations, genetic predisposition, and exposure to carcinogens play an important role in cancer progression upon viral infections.