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NATIONAL CANCER INSTITUTE (NCI): An NIH institute with the overall mission of conducting and supporting research, training and disseminating health information with respect to the causes, diagnosis and treatment of cancer. NCI also performs these functions for HIV infections and associated diseases. See also National Institutes of Health.

NATIONAL INSTITUTE OF ALLERGY AND INFECTIOUS DISEASES (NIAID): An NIH institute that conducts and supports research to study the causes of allergic, immunologic and infectious diseases, and to develop better means of preventing, diagnosing and treating illnesses. NIAID is responsible for the federally funded, national basic research program in AIDS. See also National Institutes of Health.

NATIONAL INSTITUTES OF HEALTH (NIH): A multi-institute agency of the Public Health Service, NIH is the federal focal point for health research. It conducts research in its own laboratories and supports research in universities, medical schools, hospitals and research institutions throughout this country and abroad. See also Public Health Service.

NATURAL KILLER CELLS: (NK cells). A type of lymphocyte that does not carry the markers to be B cells or T cells. Like cytotoxic T cells, they attack and kill tumor cells and protect against a wide variety of infectious microbes. They are "natural" killers because they do not need additional stimulation or need to recognize a specific antigen in order to attack and kill. Persons with immunodeficiences such as those caused by HIV infection have a decrease in "natural" killer cell activity. See also Antigen; B Lymphocytes; Cytotoxic; Lymphocyte; Null Cell; T Cells.

NCI: See National Cancer Institute.

NEURALGIA: A sharp, shooting pain along a nerve pathway.

NEUROLOGICAL COMPLICATIONS OF AIDS: See Central Nervous System (CNS) Damage.

NEUROPATHY: The name given to a group of disorders involving nerves. Symptoms range from a tingling sensation or numbness in the toes and fingers to paralysis. It is estimated that 35 percent of people with HIV disease have some form of neuropathy. A "peripheral neuropathy" refers to the peripheral nerves outside the spinal cord.

NEUTRALIZATION: The process by which an antibody binds to specific antigens, thereby "neutralizing" the microorganism. See also Antibodies; Antigen.

NEUTRALIZING ANTIBODY: An antibody that keeps a virus from infecting a cell, usually by blocking receptors on the cell or the virus. See also Antibodies; Receptor.

NEUTRALIZING DOMAIN: The section of the HIV envelope protein gp120 that elicits antibodies with neutralizing activities. See also Antibodies; gp120.

NIAID: See National Institute of Allergy and Infectious Diseases.

NIH: See National Institutes of Health.

NUCLEIC ACID: Organic substance, found in all living cells, in which the hereditary information is stored and from which it can be transferred. Nucleic acid molecules are long chains that generally occur in combination with proteins. The two chief types are DNA (deoxyribonucleic acid), found mainly in cell nuclei, and RNA (ribonucleic acid), found mostly in cytoplasm. Each nucleic acid chain is composed of subunits called nucleotides, each containing a sugar, a phosphate group, and one of four bases: adenine (symbolized A), guanine (G), cytosine (C) and thymine (T). RNA contains the sugar ribose instead of deoxyribose and the base uracil (U) instead of thymine. The specific sequences of nucleotides constitute the cell's genetic information: Each three-nucleotide DNA sequence specifies one particular amino acid. The long sequences of DNA nucleotides thus correspond to the sequences of amino acids in the cell's proteins. In order to be expressed as protein, the genetic information is carried to the protein-synthesizing machinery of the cell, usually in the cell cytoplasm. Forms of RNA mediate this process. DNA not only provides information, but also specifies its own exact replication. The cell replicates its DNA by making a complementary copy of its exact nucleotide sequence: T for every A, C for every G, G for every C, A for every T. Although the triplet nucleotide code seems to be universal, the actual sequences of the nucleotides vary according to the species and individual. See also Gene; Genetic Engineering; Mutation.

NUCLEOLI: Bodies in the nucleus that become enlarged during protein synthesis and contain the DNA template for ribosomal RNA. See also Ribonucleic Acid; Ribosome.

NUCLEOSIDE ANALOG: Nucleosides are related to nucleotides, the subunits of nucleic acids; however, they do not carry the phosphate groups of the nucleotides. Nucleoside analogs generally are synthetic compounds similar to one of the components of DNA or RNA; a general type of antiviral drug (e.g., acyclovir and AZT). See also Acyclovir; AZT; Nucleic Acid.

NUCLEUS: 1. The central controlling body within a living cell, usually a spherical unit enclosed in a membrane and containing genetic codes for maintaining the life systems of the organism and for issuing commands for growth and reproduction. 2. The nucleus of a cell is an organelle (i.e., a cellular organ) that is essential to such cell functions as reproduction and protein synthesis. It is composed of nuclear sap and a nucleoprotein-rich network from which chromosomes and nucleoli arise and is enclosed in a definite membrane. See also Nucleoli.