(single and double-toed hooved mammals). Transmission: transmitted by means not involving a vector. Geographic distribution: worldwide. Physicochemical and physical properties General Buoyant density 1.16–1.18 g cm−3 in sucrose Virions sensitive to heat, detergents, and formaldehyde Infectivity not affected by UV irradiation Classed as having class C morphology Nucleic acid Virions contain 2% nucleic acid Genome consists of a dimer Virions contain one molecule of (each) linear positive-sense single stranded RNA. Total genome length is of one monomer ranges from 8k-10k nt (depending on the virus). Genome sequence has terminal repeated sequences; long terminal repeats (LTR) (of about 600 nt) The 5' end of the genome has a cap Cap sequence of type 1 m7G5ppp5'GmpNp 3' end of each monomer has a poly (A) tract. 2 copies packed per particle (held together by Watson-Crick baseparing to form a dimer). There are 11 proteins Virions contain 60% protein Five (major)structural virion proteins have been found so far Lipids: Virions contain 35% lipid. Carbohydrates: Other compounds detected in the particles 3% carbohydrates. Use as gene delivery vectors Lentivirus is primarily a research tool used to introduce a gene product into in vitro systems or animal models. Large-scale collaborative efforts are underway to use lentiviruses to block the expression of a specific gene using RNA interference technology in high-throughput formats. Conversely, lentivirus are also used to stably over-express certain genes, thus allowing researchers to examine the effect of increased gene expression in a model system. Another common application is to use a lentivirus to introduce a new gene into human or animal cells. For example, a model of mouse hemophilia is corrected by expressing wild-type platelet-factor VIII, the gene that is mutated in human hemophilia. Lentiviral infection has advantages over other gene-therapy methods including high-efficiency infection of dividing and non-dividing cells, long-term stable expression of a transgene, and low immunogenicity. Lentiviruses have also been successfully used for transduction of diabetic mice with the gene encoding PDGF (platelet-derived growth factor), a therapy being considered for use in humans. Finally, lentiviruses have been also used to elicit an immune response against tumor antigens. These treatments, like most current gene therapy experiments, show promise but are yet to be established as safe and effective in controlled human studies. Gammaretroviral and lentiviral vectors have so far been used in more than 300 clinical trials, addressing treatment options for various diseases. See also Visna virus Notes References Further reading