RANDOMIZED TRIAL: A study in which participants are randomly assigned to either a treatment arm or placebo arm of a clinical trial. See also Clinical Trial; Placebo.

RECEPTOR: A molecule on the surface of a cell that serves as a recognition or binding site for antigens, antibodies or other cellular or immunological components. See also Antibodies; Antigen.

RECOMBINANT: An organism whose genome contains integrated genetic material from a different organism. See also Genome.

RECOMBINANT DNA: See Biotechnology; Genetic Engineering.

REGULATORY T CELLS: T cells that direct other immune cells to perform special functions. The chief regulatory cell, the CD4+ T cell or T helper cell, is HIV's chief target. See also CD4 (T4) or CD4+ Cells; T Cells.

REMISSIONS: The lessening of the severity or duration of outbreaks of a disease, or the abatement (diminution in degree or intensity) of symptoms altogether over a period of time.

RETROVIRUS: HIV and other viruses that carry their genetic material in the form of RNA and that have the enzyme reverse transcriptase. Like all viruses, HIV can replicate only inside cells, commandeering the cell's machinery to reproduce. Like other retroviruses, HIV uses the enzyme called reverse transcriptase to convert its RNA into DNA, which is then integrated into the host cell DNA. See also DNA; Reverse Transcriptase; Ribonucleic Acid.

REV: One of the regulatory genes of the HIV virus. Three HIV regulatory genes-tat, rev and nef-and three so-called auxiliary genes-vif, vpr and vpu-contain information necessary for the production of proteins that control the virus's ability to infect a cell, produce new copies of the virus or cause disease. See also nef; tat.

REVERSE TRANSCRIPTASE: This enzyme of the HIV virus (and other retroviruses) converts the single-stranded viral RNA into DNA, the form in which the cell carries its genes. The antiviral drugs approved in the US for the treatment of HIV infection-AZT, ddC and ddI-all work by interfering with this stage of the viral life cycle. See also AZT; ddC; ddI; DNA; Ribonucleic Acid.

RIBONUCLEIC ACID (RNA): 1. A nucleic acid, found mostly in the cytoplasm of cells, that is important in the synthesis of proteins. The amount of RNA varies from cell to cell. RNA, like the structurally similar DNA, is a chain made up of subunits called nucleotides. In protein synthesis, messenger RNA (mRNA) replicates the DNA code for a protein and moves to sites in the cell called ribosomes. There, transfer RNA (tRNA) assembles amino acids to form the protein specified by the messenger RNA. Most forms of RNA (including messenger and transfer RNA) consist of a single nucleotide strand, but a few forms of viral RNA that function as carriers of genetic information (instead of DNA) are double-stranded. 2. A nucleic acid associated with the control of chemical activities inside a cell. One type of RNA transfers information from the cell's DNA to the protein-forming system of a cell outside the nucleus. Some viruses (e.g., HIV) carry RNA instead of the more usual genetic material DNA. See also Cytoplasm; DNA; Retrovirus.

RIBOSOME: A cytoplasmic organelle composed of ribonucleic acid and protein that functions in the synthesis of protein. Ribosomes interact with messenger RNA and transfer RNA to join together amino acid units into a polypeptide chain according to the sequence determined by the genetic code. See also Cytoplasm; Ribonucleic Acid.

RNA: See Ribonucleic Acid.