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One of the obstacles to treatment of the Human Immunodeficiency Virus is its high genetic variability.[1] HIV can be divided into two major types, HIV type 1 (HIV-1) and HIV type 2 (HIV-2). HIV-1 is related to viruses found in chimpanzees and gorillas living in western Africa, while HIV-2 viruses are related to viruses found in sooty mangabeys. HIV-1 viruses may be further divided into groups. The HIV-1 group M viruses predominate and are responsible for the AIDS pandemic. Group M can be further subdivided into subtypes based on genetic sequence data. Some of the subtypes are known to be more virulent or are resistant to different medications. Likewise, HIV-2 viruses are thought to be less virulent and transmissible than HIV-1 M group viruses, although HIV-2 is known to cause AIDS.
[edit] Major Types[edit] HIV-1HIV-1 is the most common and pathogenic strain of the virus. Scientists divide HIV-1 into a Group M (major) and two or more minor groups.
With 'M' for "major", this is by far the most common type of HIV, with more than 90% of HIV/AIDS cases deriving from infection with HIV-1 group M. The M group is subdivided further into clades, called subtypes, that are also given a letter. There are also "circulating recombinant forms" or CRFs derived from recombination between viruses of different subtypes which are each given a number. CRF12_BF, for example, is a recombination between subtypes B and F.
These subtypes are sometimes further split into sub-subtypes such as A1 and A2 or F1 and F2.[citation needed] This is not thought to be a complete or final list, and further types will likely be found.[6] 
The 'N' stands for "non-M, non-O". This group was discovered in 1998 and has only been seen in Cameroon. As of 2006, only 10 Group N infections had been identified.[7]
The O ("Outlier") group is not usually seen outside of West-central Africa. It is reportedly most common in Cameroon, where a 1997 survey found that about 2% of HIV-positive samples were from Group O.[8] The group caused some concern because it could not be detected by early versions of the HIV-1 test kits. More advanced HIV tests have now been developed to detect both Group O and Group N.[9]
In 2009, a newly-analyzed HIV sequence was reported to have greater similarity to a simian immunodeficiency virus recently discovered in wild gorillas (SIVgor) than to SIVs from chimpanzees (SIVcpz). The virus had been isolated from a Cameroonian woman residing in France who was diagnosed with HIV-1 infection in 2004. The scientists reporting this sequence placed it in a proposed Group P "pending the identification of further human cases".[10][11] [edit] HIV-2Like HIV-1 Groups N and O, HIV-2 has not been widely seen outside of Africa. The first case in the United States was in 1987.[12] Many test kits for HIV-1 will also detect HIV-2[13]. [edit] Drug Resistance MutationsIsolates of HIV-1 and HIV-2 with resistance to antiretroviral drugs arise through genetic mutations, which have been tracked and analyzed. The Stanford HIV Drug Resistance Database and the International AIDS Society publish lists of the most important of these; first year listing 80 common mutations, and the latest year 93 common mutations, and made available through the Stanford HIV RT and Protease Sequence Database. [edit] See also[edit] References
[edit] External links
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