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the best way to use stavudine (as well as other HIV medications) is in combination with other drugs. It is often combined with zidovudine or lamivudine and a protease inhibitor. The two-drug combination of zidovudine and stavudine alone is not recommended, as they do not work well together. As the years have passed since the introduction of stavudine, several side effects have been connected to its use. peripheral neuropathy is the most common. Doses of stavudine can be lowered somewhat if doing so decreases the symptoms; otherwise a change in drugs is recommended. Pancreatitis (inflammation of the pancreas) has occurred with stavudine as well as elevated liver enzyme levels. Both of these organ functions should be checked regularly when taking stavudine. Fatal lactic acido-sis has occurred in pregnant women receiving the combination of didanosine and stavudine. Although it is not yet certain, some researchers have found that long-term use of stavudine may be a risk factor for some of the symptoms of lipodystrophy, such as loss of tissue from the face.

STD See sexually transmitted disease.

steatosis The collection of excessive amounts of fats inside liver cells, which can also be referred to as fatty liver. By itself the condition is not threatening, though it usually indicates other problems. Causes of steatosis include hepatitis, alcoholism, malnutrition, pregnancy, and drug toxicities. Treatment involves eliminating the cause of the fat buildup, and prognosis is good if the condition is recognized and the problem has not continued over many years.

In HIV-positive people steatosis is one of the results attributed to mitochondrial toxicity that has occurred after lengthy periods on NRTIs or reactions to NRTI drugs.

stem cell The cells of which all immune cells are descendants; found in the bone marrow in small pools, stem cells multiply and mature, when needed, into functional CD4+, t and other white and red blood cells. Stem cells have often been thought to be the best reservoirs for harboring anti-HIV genes. They have been notoriously difficult to identify and purify, but a newly developed simple method for isolating and maintaining them in culture dishes has enabled new research.

Researchers are currently trying to insert disease-resistant genes into the body's blood-forming cell "factories" to help cells counter HIV infection. The goal is to be able to reconstitute the immune system of HIV-infected patients with genetically modified HIV-resistant T cells. To deliver the therapeutic genes to the stem cells, the genes are enclosed in a harmless virus, an engineered adeno-associated virus (AAV). One of the genes being used makes T cells HIV-resistant through coding for the intracellular production of antibodies against HIV. In laboratory experiments, AAV has effectively transported such ANTiBODY-coding genes into test-tube grown cells, and the antibody produced inside these cells has successfully blocked HIV growth. Researchers intend to carry out these same experiments using stem cells isolated from fragments of HIV-positive patients' bone marrow. Eventually, the HIV-resistant stem cells will be returned to the patients, where they are expected to "home" back to the bone marrow and, as needed, multiply and develop into mature immune cells that are resistant to HIV.

Researchers are also trying to combine different methods of gene therapy against HIV. One strategy is to deliver different types of anti-HIV genes at the same time, all capable of inhibiting HIV growth by blocking either the function of one HIV gene or the function of one of the virus's protein components. Each kind of anti-HIV gene targets a single but critical step in the HIV's life cycle.

Because HIV inserts its genes into its host cell's own genetic material, which other viruses such as those that cause cold, flu, and herpes do not do, it functions, in a sense, as a contagious genetic disease. This is one of the reasons why strategies such as gene therapy, which are typically applied to inherited single-gene abnormalities, are so attractive as potential anti-HIV therapeutics.

sterility Inability of the female to become pregnant or for the male to impregnate a female; also, condition of being free from living microorganisms.

sterilization The destruction of all microorganisms in, on, or about an object by employing various

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