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Re: HIV is not the only exogenous retrovirus.

I have no idea what Jeffrey is talking about here. There are dozens of exogenous retroviruses which have been characterized in detail:
HTLVs, STLVs and a few other T-cell leukemia viruses are certainly exogenous. They, like the lentiviruses are "complex" retroviruses with regulatory genes in addition to the Gag-Pol-Env genes found in all retroviruses. So far, nobody has ever found a complex retrovirus in an endogenous state (found in the germ-line DNA).

There are many exogenous retroviruses of livestock and fowl that are of economic importance in the agricultural industry. The Avian Leukosis/Sarcoma Viruses are just one example. Look up all the research that has been done in recent years on the "J subtype" of ALV for an introduction.

Lupiani B, Williams SM, Silva RF, Hunt HD, Fadly AM.
Pathogenicity of two recombinant avian leukosis viruses.
Avian Dis. 2003 Apr-Jun;47(2):425-32.
PMID: 12887202

Wang CH, Juan YW.
Occurrence of subgroup J avian leukosis virus in Taiwan.
Avian Pathol. 2002 Oct;31(5):435-9.
PMID: 12427337

Landman WJ, Post J, Boonstra-Blom AG, Buyse J, Elbers AR, Koch G.
Effect of an in ovo infection with a Dutch avian leukosis virus subgroup J isolate on the growth and immunological performance of SPF broiler chickens.
Avian Pathol. 2002 Feb;31(1):59-72.
PMID: 12425793

ALV-J appears to be a recent recombinant between the exogenous Rous Sarcoma Virus type of ALV, and an endogenous fowl retrovirus. Below is an example of a bovine exogenous retrovirus that has been less well characterized than ALV-J:

Nuotio L, Rusanen H, Sihvonen L, Neuvonen E.
Eradication of enzootic bovine leukosis from Finland.
Prev Vet Med. 2003 May 30;59(1-2):43-9.
PMID: 12719016

Competing interests:   None declared