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Adherence
The degree to which a patient exactly follows a prescribed treatment regimen. Poor adherence may negatively impact a drug's effectiveness. Compliance is an alternate term.

Amino Acid
A nitrogen-containing molecule that serves as a building block for proteins, including enzymes, muscles, and structural molecules. The human body uses twenty of the eighty amino acids found in nature.

Antiretroviral
A substance that stops or suppresses the activity of a retrovirus such as HIV. AZT, ddC, ddI, and d4T are examples of antiretroviral drugs. Antiviral is sometimes used as an alternate term.

Assay
A test.

bDNA (branched DNA)
A test for measuring the amount of HIV and other viruses in the blood. Test results are reported in numbers of virus particle equivalents per milliliter of plasma. See also PCR.

Codon
A three-nucleotide genetic subunit that determines which amino acid is placed at one point in a protein chain. Mutations at specific HIV codons are associated with changes in the amino acid sequence of HIV's proteins and enzymes. Such mutations can cause HIV to become resistant to antiretroviral drugs.

Cross-Resistance
The phenomenon by which HIV and other disease-causing organisms become resistant to more than one drug after a single therapy. For example, people who develop resistance from taking one non-nucleoside reverse transcriptase inhibitor (NNRTI) are likely to be cross-resistant to other drugs in the same class.

DNA (Deoxyribonucleic Acid)
A double-stranded molecule that makes up the chromosomes in the center of a cell and that carries genetic information in the form of genes.

Enzyme
A cellular protein whose shape allows it to hold together several other molecules in close proximity to each other. Enzymes also induce chemical reactions in other substances.

First-Line Treatment
The best starting therapy for someone who has never received therapy before. Because of the potential for the development of cross-resistance by HIV and other microbes, the choice of first-line medication(s) affects the efficacy of subsequent therapies.

Gene
A unit of DNA in the chromosomes that determines the structure of a specific protein or enzyme. Genes regulate the metabolism of individual cells and the development and specialization of body cells and tissues.

Genotype
The genetic makeup of an individual organism, determined by the sequence of nucleotides in its genes. See also Phenotype.

Genotypic Assay
A blood test that determines the genetic sequences of an organism. Frequently performed in HIV to establish whether certain mutations conferring drug resistance are present. See also Phenotypic Assay; Resistance.

HAART (Highly Active Antiretroviral Therapy)
Potent antiretroviral treatment usually including a combination of three or more drugs whose purpose is to reduce viral load to undetectable levels.

IC (Inhibitory Concentration)
The amount of drug in the blood needed to suppress the reproduction of a disease-causing microorganism such as HIV. For example, IC95 is the drug level needed to block 95% of HIV's normal replication; IC50 is the drug level needed to block 50% of HIV's normal replication.

NNRTI (Non-Nucleoside Reverse Transcriptase Inhibitor)
A member of a class of compounds -- including efavirenz, delavirdine, and nevirapine -- that acts directly to combine with and block the action of HIV's reverse transcriptase to prevent viral RNA from being converted into DNA and integrated into the uninfected cell's nucleus. NNRTIs have suffered from HIV's ability to mutate rapidly and become resistant to their effects.

Nucleoside
The molecular units that serve as the building blocks of DNA and RNA, the genetic material found in living organisms.

Nucleoside Analog
A type of antiviral drug, such as AZT, ddI, ddC, d4T, 3tc, or abacavir whose structure constitutes a defective version of a natural nucleoside. Like NNRTIs, these drugs block the viral enzyme responsible for converting HIV RNA into DNA, ultimately preventing the cell from becoming infected.

PCR (Polymerase Chain Reaction)
A sensitive test that amplifies DNA. PCR is a critical part of tests for viral load, genotyping, and phenotyping.

Phenotype
The functional capabilities and outward appearance of a microorganism. It is the physical expression of the genotype.

Phenotypic Assay
A test that measures the sensitivity of HIV to specific antiretroviral drugs. It is considered more of a direct measure of HIV drug resistance than genotypic tests. See also Genotypic Assay.

Protease
An enzyme that triggers the breakdown of proteins. HIV's protease enzyme breaks apart long strands of viral protein into the separate proteins constituting the viral core and the enzymes it contains. HIV protease acts as new virus particles are budding off a cell membrane.

Protease Inhibitor
A drug that binds to and blocks HIV protease from working, thus preventing the production of new functional viral particles. Examples include saquinavir, ritonavir, indinavir, nelfinavir, and amprenavir.

Protein
Large molecules made up of long sequences of amino acids. Some hormones and all enzymes and cellular structural components are proteins.

Resistance
Reduction in an organism's sensitivity to a particular drug. Resistance is thought to result mainly from a genetic mutation. In HIV, such mutations can change the structure of viral enzymes and proteins so that an antiviral drug can no longer bind with them. Resistance detected by searching a pathogen's genetic makeup for mutations believed to confer lower susceptibility is called genotypic resistance. Resistance found by successfully growing laboratory cultures of the pathogen in the presence of a drug is called phenotypic resistance.

RNA (Ribonucleic Acid)
A single-stranded molecule composed of nucleotide sequences. It is similar in basic structure to half of the double-stranded DNA. In cells, RNA transmits the code from the DNA-based genes that instruct the cells' chemical machinery to produce structural proteins and enzymes. In retroviruses, RNA is the sole repository of the viral genes.

Sensitivity
The degree to which an organism is affected by a drug. See also Resistance.

Wild-Type Virus
Naturally occurring HIV with an optimal genetic makeup and no laboratory-induced mutational defects. The term also refers to HIV that has not been exposed to antiviral drugs and therefore has not accumulated mutations conferring drug resistance.