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What Is a Retrovirus?

A retrovirus belongs to a group of RNA viruses that insert a DNA copy of their genome into a host cell and replicates.

Different Types of Retroviruses

Human Retroviruses

These are the types that affect humans most commonly. There are six human retroviruses that have been identified so far. These include HTLV-1, HTLV-2, HIV-1, HIV-2, HTLV-3, and HTLV-4 (recently identified). One of the most common examples of a retrovirus is the human immunodeficiency virus (HIV). Other examples include the human T-cell lymphotropic virus type 1 (HTLV-1). This virus can cause a type of cancer, which is called adult T-cell leukemia.


This is a type of retrovirus that contains oncogenes and is associated with tumor development. However, this type is not known to infect humans.

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Defective Retrovirus

This type has lost a gene that is critical for replication and therefore cannot cause infection without a helper virus.

Endogenous Retrovirus

This type of retrovirus integrates itself into the germ cell DNA. Even though nearly 11% of the human genome is made of the endogenous sequence, the virus is primarily defective.

Retroviruses exist in humans in two forms. One is in the form of normal genetic elements and others as horizontally-transmitted infectious RNA-containing viruses that are typically transmitted from human to human (e.g., HIV and HTLV). The human T-cell leukemia virus is known to be associated with some very rare diseases in humans. These infections are generally presented in the form of asymptomatic infection, adult T-cell leukemia, and tropic spastic paraparesis. The HIV virus presents itself in three forms as well: asymptomatic infection, acute infection, and acquired immune deficiency syndrome.

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