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A family of RNA viruses 80-100 nm in diameter, enveloped, and containing two identical molecules of positive sense, single-stranded RNA, molecular weight 3-6 × 106; genomic RNA serves as a template for the synthesis of a complementary DNA, which may be integrated into the host DNA. At present, seven genera have been identified: Mammalian type B retroviruses, Mammalian type C retroviruses, Avian type C retroviruses, Type D retroviruses, BLV-HTLV retroviruses, Lentivirus, and Spumovirus.
A family of viruses grouped in three subfamilies: Oncovirinae (HTLV-I, HTLV-II RNA tumor viruses), Spumavirinae (foamy viruses), and Lentivirinae (HIV-like viruses, visna, and related agents).
a family of medium-sized (about 100 nm diameter) enveloped viruses with an icosahedral capsid enclosing the single-strand RNA genome which is diploid, i.e. two copies per virion. There are three subfamilies: Oncornavirinae, which includes avian, bovine, feline and murine leukemia/sarcoma viruses; Lentivirinae, which includes maedi/visna virus of sheep, caprine arthritis encephalitis and equine infectious anemia viruses and human immunodeficiency virus 1 and 2; Spumavirinae, which includes nonpathogenic viruses of monkey, cattle and cats, recognized only in cell culture where they produce syncytia (bovine and feline syncytia-forming viruses) which have a vacuolated cytoplasm, hence also called foamy viruses.