The classification of retroviruses is based on comparisons of the size of the genome and morphologic characteristics (see Table 1). The genomic RNA
|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|
|Lentivirus||genome > bar-shaped||human immunodeficiency virus||humans||immunodeficiency and|
|concentric core||neurologic disease|
|Spumavirus||assembly as intracyto-||chimpanzee foamy spumavirus||simians||none apparent|
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.