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The classification of retroviruses is based on comparisons of the size of the genome and morphologic characteristics (see Table 1). The genomic RNA Table 1. Viruses of the family Retroviridae.

Genus Distinguishing feature Example Host Diseases/pathologies
Alpha-retrovirus genome <8kb; assembly at cell membrane avian leukosis virus birds malignancies
Beta-retrovirus intracytoplasmic assembly mouse mammary tumor virus mice mammary and ovarian
(B- or D-type) carcinoma; lymphomas
Gamma-retrovirus genome < assembly murine leukemia virus mice malignancies
at cell membrane
Delta-retrovirus genomes < C-type bovine leukemia virus cows malignancies
Epsilon-retrovirus assembly at cell membrane; walleye dermal sarcoma virus fish solid tumors
hosts: fish
Lentivirus genome > bar-shaped human immunodeficiency virus humans immunodeficiency and
concentric core neurologic disease
Spumavirus assembly as intracyto- chimpanzee foamy spumavirus simians none apparent
plasmic particles

of retroviruses is single-stranded and possesses "positive" polarity similar to that found in messenger RNA (mRNA). Virions (virus particles) contain two 5′ ("five prime"), end-linked, identical copies of the genome RNA, and are therefore said to be diploid.

Three genes are universally present in the genomes of retroviruses that are capable of replication, such as murine (mouse) leukemia virus. The gag (group antigen) gene encodes proteins that make up the nucleocapsid of the virus as well as a matrix layer, the two of which surround the RNA. The pol gene (a type of polymerase) encodes reverse transcriptase, which copies the RNA into DNA, and integrase, which integrates the DNA into the host chromosome. Depending on the species, pol can also encode protease, a protein that cleaves the initial multiprotein products of retrovirus translation to make functional proteins. Some retroviruses have incorporated viral oncogene sequences. An example of this is reticuloendotheliosis virus strain T. The genome of complex retroviruses, such as HTLV, can contain several other genes that regulate genome expression or replication and are not present in simple retroviruses.

Additional topics

Medicine EncyclopediaGenetics in Medicine - Part 4Retrovirus - Characteristics, Reverse Transcriptase, Retrovirus Replication Cycle, Consequences Of Retroviral Infection, Retroviruses And Cancer